Monday, January 31, 2011

The Extended Range


Below is a link to the NWS Hydrometeorological Prediction Center’s (HPC) extended range forecast discussion. Although it is technical, it serves as a good representation of what really goes on in weather forecasting/predicting/guessing 72+ hrs onwards. If you read it, you will note that most of the discussion written by the meteorologist centers on forecast models and their behaviors/biases/handling of major weather features and the impact in terms of sensible weather that could result. These are the 5 models primarily used for extended range forecasting :
  • GFS (US)
  • ECMWF (European)
  • CMC (Canadian)
  • UKMET (UK)
  • JMA (Japanese)
 A GFS Ensemble run uses the same data input into the operational/deterministic model, but differs because instead of 1 (operational) forecast, an "ensemble" is 20 different calculations (perturbations). Essentially they are “what if” scenarios - when the ensembles are similar, the model is signalling a higher degree of accuracy. Here is a graphical representation of an ensemble run from earlier today at forecast hour 126 which would be next Sat I believe. The picture (Click to Enlarge) shows 6hr Precip, 850mb Temps, and Pressure.
Sunday Jan 30 18Z GFS Ensemble - Forecast Hr 126*


 *From Penn State's Meteorology E-Wall, found here: http://www.meteo.psu.edu/ewall/ewall.html

Essentially - granted this is 5 days away - the ensembles agree that there is going to be a low pressure system forming in/near the Gulf of Mexico. But as far as timing, critical thickness temperatures (rain vs snow, etc), and amount of precipitation we can expect here is impossible to predict with any certainty. This is the reason behind the National Weather Service's use of percentage probabilities for a given event. A 50% chance of rain doesn't mean maybe yes, maybe no. It is the probability of a given area within one of their forecast zones (or subzones) of rain falling. If you look at the NWS forecast for Salisbury on Saturday the probabilities are somewhere around 30% or thereabouts. All the major models are indicating that a storm will be in the making or occurring around Saturday; but with 120 hrs until potential impact the chances of model correction in the time/speed/strength of the low pressure system that doesn't even exist yet are way too high to place the probability above that. This is especially true when it comes to predicting winter weather events in the NC Piedmont because we are usually the borderline between the cold to the North and warmth from the South associated with low pressure systems moving north from the gulf region. Perhaps one day a computer will spit out a perfect prediction for the weather in your backyard five days from now; we simply need someone to write a flawless algorithm or two...

Tangent aside, do read the discussion - an actual meteorologist analyzing model data and accuracy - and while computer modeling is (in my opinion) the #1 forecasting tool - you will see that is not all he discusses in his extended range discussion.

Jump to Discussion:  http://www.hpc.ncep.noaa.gov/discussions/pmdepd.html 

Last note - since today is another good example of the impact cold air damming has on our weather I am going to try to do a post about it if I can find some info I can link to. Enjoy the gray for a few days.

Monday, January 24, 2011

Non-News Flash: Rain Headed Our Way...Snow Possible at the End?

Soaking rain starts late tomorrow and continues through Wednesday afternoon. Precipitation amounts approaching 1" area wide. Should warm up into the 40's tomorrow as rain doesn't begin in earnest until later in the PM. Wednesday looks to be delightful with heavy overcast, rain, and temperatures around 40.

With that said, the last vestige of hope for fellow snow lovers out there is the scenario I alluded to in my previous post - the passage of the mid/upper level low troughs as the surface storm begins to pull away/up the coast - could bring a departing change over from rain to snow. This could happen (although it is rare in the west-central NC Piedmont) if the upper lows pass near enough, allowing cold air to be pulled close enough to the surface to overcome the warming that will have occurred between approx the 1000-800mb level.

This is the whole reason the storm is going to be a cold rain...warm air in the mid levels. If you want to look at Skew-T representations/explanations, go back a few posts and read the rain/snow section.

So what if? Ultimately, nothing major that I can foresee at this point. The latest runs of the European/NAM/and GFS all indicate the upper level lows will pass in our vicinity. If they are close, we could see a 6 hour period (Wed Evening/Night) of a rain snow mix changing to snow and accumulating on grassy surfaces. If we get to freezing overnight before streets dry, we could see icy spots Thursday morning.

I'll keep you updated.

Sunday, January 23, 2011

Not quite DOA...but potential for significant snow storm rapidly decreasing

Low pressure system still on track to bring lots of moisture to the area beginning late Tuesday into Wednesday but unfortunately the timing delay and prognosticated track of the system means potential for snow waning.

As always with winter storms in this region, it will be the last minute details - track, timing, and strength - that determine if any white stuff falls. Best chance for snow now looks to be at the END of the storm as the coastal low begins to wrap up and move away from us, bringing mid and lower levels temperatures down to marginal levels for a switchover from rain to snow. I'll try to post in more detail tomorrow. Stay tuned...

Friday, January 21, 2011

Storm Clouds on the Horizon?

Quick thoughts (odd 3rd person interview really) concerning the potential winter storm next week

Low Pressure - Check

Origin - The Gulf of Mexico

Affecting Our Area - Check

Precipitation From Said Low Pressure System - Check

Location of Low Pressure - Near the NC Coastline 

Miller-A or Miller-B Type Low? - Miller A (Coastal Low Only)

Amount of Precipitation - Hmmm...best guess is ~.75" but that's a shot in the dark right now

Form of Precipitation - Harder question, but leaning towards snow

Cold Air - High Pressure to our North over New England

Timing - Seriously? Well, right now I'd say it will begin at exactly 2:29 pm on Tuesday and end sometime on Wednesday.

Amount of Snow on the Ground in Salisbury When Storm Passes - Funny. I honestly have no idea right now. Somewhere between none and 12"

Odds of Being Correct - somewhere around 50%

Next Post - Most Likely Saturday or Sunday. 

Weather for Next Week in General - Dry Monday. Storm Tuesday into Wednesday. Below Normal Temperatures and Dry Otherwise.

Favorite Blogging Style - This. Definitely. But, I'll try not to do it again.

Stay Tuned....

Thursday, January 20, 2011

Winter Weather 101: The Factors(s) That Determine Precipitation Types

WARNING: Nerdy weather-ness ahead with some low quality Skew-T diagrams and plenty of technical terms (actually included this time)

If you are the type who upon occasions find yourself staring blankly ahead thinking of the possible atmospheric conditions required for snow, sleet, or freezing rain, and the differences/requirements for one versus type versus another, read on my friend....

Quick Notes:
A sounding is a measure of several variables in a column of air - Temperature, Dewpoint, Pressure, and Wind - at different heights (ft/millibars). This is what "weather balloons" measure. Technically they are called RAOBs and are launched twice daily from select locations. Greensboro is the closest/best for our area.

A Skew-T diagram (see below) is simply a visual representation of the observed data from the balloon. The name refers to the temperature lines - they are "skewed" to the right. So, the right solid line is the observed temperature and the left dashed line is the dewpoint.

This following is edited and copied from NWS Louisville's Forecast Office. 
Click the title to jump to their page.

"PRECIPITATION CONCEPTS; FACTORS AFFECTING PRECIPITATION"

Freezing Rain:
Precipitation starts in cold air aloft as ice crystals/snowflakes that have formed via heterogeneous nucleation, deposition, and ice multiplication and grown through aggregation and riming. Crystals then fall through a melting (warm) layer that is sufficiently deep or warm enough to completely melt the crystals to water drops. The drops then become supercooled as they fall through a subfreezing (refreezing) layer near the surface and freeze on contact if ground objects are colder than 0 C (32 F). A typical freezing rain sounding is shown in Figure 2.

Melting layers greater than 1200 ft deep usually cause complete melting, although for large flakes or for maximum melting layer temperatures less than or equal to 1 C, a deeper melting layer may be required for complete melting.

Figure 2

If the ground temperature is warmer than 0 C but the air temperature is colder than 0 C, then heat conduction may prevent freezing on the ground/streets, but freezing will occur on elevated cold surfaces, i.e., trees, power lines, and cars. If the ground is frozen, then freezing rain can occur despite air temperatures above 0 C (at least for awhile).

Sleet:
A sleet sounding is similar to that for freezing rain, i.e., a melting layer aloft and subfreezing layer below are present. The main differences are the depth and temperature of these two layers and the crystal/snowflake size as to whether complete (freezing rain) or partial (sleet) melting occurs. A typical sleet sounding is shown in Figure 5. Partially melted snowflakes can refreeze much more readily resulting in sleet (or even snow) than completely melted crystals, since ice nuclei still exist in partially melted particles.

As the melting layer decreases in depth and temperature and the subfreezing layer increases in depth, the probability of sleet or a mix of sleet and snow graupel increases.

Figure 5

Unmixed (no freezing rain) sleet events usually are most probable when the average minimum temperature of the low-level subfreezing layer is colder than -5 C. Mixed (sleet and freezing rain) events can occur when the minimum temperature of the subfreezing layer is as warm as -2.5 C.

Snow Versus Rain:
A borderline rain/snow sounding (Figure 6) is different (opposite) from freezing rain and sleet soundings in that the melting/warm layer usually exists near the surface while subfreezing temperatures are located aloft above the warmer boundary layer air.

The depth of low-level warm air (below the freezing level) needed to melt snow falling from above to rain varies from about 750-1500 ft and depends on the mass of the flakes and the lapse rate of the melting layer. When the lapse rate is small (i.e., temperature decreases slowly with height in the layer; left diagram in Figure 7), the melting layer is weak so it must be deeper to melt snow completely. When the lapse rate is large (right diagram in Figure 7), the layer can be smaller and still melt snow.
Figure 6 Figure 7
Precipitation likely will be snow at the surface if the height/depth of the melting layer is less than 900 ft (i.e., a 50 percent or greater chance of snow reaching the ground). If the depth is only about 200 ft (i.e., surface temperatures are above freezing but the freezing level is at 200 ft above the ground), then the probability of snow is about 90 percent. If the depth is more than 1000 ft, then the probability of snow decreases rapidly below 50 percent.

It is crucial to evaluate the depth and temperature of warm and cold layers (as well as which layer lies on top versus underneath) in soundings to help determine precipitation type in the winter.

That's it for now. If you have any questions or would like me to (try to) explain certain terms/ideas in greater detail, let me know.

Wednesday, January 19, 2011

Looking Ahead (With Lots of Technical Terms)

Wednesday 1/19/11

We (and most of the US) are in an active weather pattern and all signs point to this continuing for the next 7-14 days at least. So what does this mean? We're in an active weather pattern - already went over that...

One might ask - what is an active weather pattern? It means there is a whole lotta weather going on. Everywhere. But this is true everyday...

So in summation - active weather pattern - which means the chances for snow are better than average - but that's only because the chance of precipitation is higher than average...


Tomorrow Night = maybe (10% chance) a few wet flakes - doubt we see much precipitation of/in any form at all.



Storm To Watch = Monday/Tuesday - not looking good at this point snowfall wise...stay tuned

Wednesday, January 12, 2011

Jan 10-11 Storm Snowfall Map

Courtesy of the NWS in Raleigh:

Click To Enlarge

Anybody notice the accumulations in the southeast tip of the county? I'm gonna have to count that as verification of my forecast...

Monday, January 10, 2011

Storm Update

Clearly not going to make the 5" snow mark. But, it's not over yet. Expect snow showers to continue this evening, changing to a snow/sleet mixture, and then freezing rain/drizzle overnight - additional accumulations highly dependent upon exactly where the showers pass. See radar shot below (moisture moving NE).

Greenville Radar at 6:51pm EST


No major ice but enough to make everything slick. Temperatures won't rise above freezing until tomorrow around noon or later, so unless roads are treated, expect them to remain slick and snow/sleet covered. Here are the latest reports from around the area, courtesy of the NWS. Rowan has generally received 2-3" from what I've seen. Please leave a comment on how much snowfall you have received and where.

452 PM EST MON JAN 10 2011

THE FOLLOWING ARE UNOFFICIAL OBSERVATIONS TAKEN DURING THE PAST 5 HOURS
FOR THE STORM THAT HAS BEEN AFFECTING OUR REGION.  APPRECIATION IS EXTENDED
TO HIGHWAY DEPARTMENTS...COOPERATIVE OBSERVERS...SKYWARN SPOTTERS
AND MEDIA FOR THESE REPORTS.  THIS SUMMARY IS ALSO AVAILABLE ON OUR
HOME PAGE AT WEATHER.GOV/GSP

********************STORM TOTAL SNOWFALL********************

LOCATION          STORM TOTAL     TIME/DATE   COMMENTS
                     SNOWFALL           OF
                     /INCHES/   MEASUREMENT

NORTH CAROLINA

...ALEXANDER COUNTY...
   TAYLORSVILLE           2.0   223 PM  1/10  DOWNTOWN TAYLORSVILLE

...BUNCOMBE COUNTY...
   3 SW CANTO             8.0   421 PM  1/10
   6 NE CANTON            7.0   220 PM  1/10  0.5 INCH  SWE
   2 NW FLETCHER          6.8   400 PM  1/10  ASHEVILLE AIRPORT

...DAVIE COUNTY...
   2 SW SHEFFIELD         0.5   346 PM  1/10
   MOCKSVILLE             0.3   320 PM  1/10

...GASTON COUNTY...
   3 NW CROWDERS          5.0  1213 PM  1/10

...HENDERSON COUNTY...
   1 W FLETCHER           6.6   100 PM  1/10

...IREDELL COUNTY...
   TROUTMAN               3.0   333 PM  1/10

...JACKSON COUNTY...
   1 NNW CASHIERS        12.5   130 PM  1/10

...MECKLENBURG COUNTY...
   4 S PAW CREEK          3.9   400 PM  1/10

...POLK COUNTY...
   5 SE COLUMBUS          4.8  1250 PM  1/10

...RUTHERFORD COUNTY...
   BOSTIC                 6.0   156 PM  1/10
 
NORTH CAROLINA (RDU)

...ANSON COUNTY...
   2 S WADESBORO          6.5   112 PM  1/10  ROADS COVERED
   WADESBORO              5.0   909 AM  1/10

...CUMBERLAND COUNTY...
   VANDER                 3.5   550 PM  1/10
   FAYETTEVILLE           3.0  1000 AM  1/10
   3 NW FAYETTEVILLE      3.0   126 PM  1/10  SNOW RATE OVER INCH HR
   SPRING LAKE            0.5   705 AM  1/10

...DAVIDSON COUNTY...
   DENTON                 3.0   500 PM  1/10
   HIGH ROCK              3.0   813 AM  1/10  DAVIDSON ROWAN LINE
   5 SW LEXINGTON         1.0   842 AM  1/10  ROADS COVERED
   LEXINGTON              0.1   525 AM  1/10  DUSTING.

...GUILFORD COUNTY...
   1 WNW GREENSBORO       0.2   430 PM  1/10
   5 ESE COLFAX             T   430 PM  1/10  TRACE AT KGSO

...HARNETT COUNTY...
   ANDERSON CREEK         2.0   352 PM  1/10
   DUNN                   0.5   658 AM  1/10
   4 SSW ANGIER           0.3   421 PM  1/10

...HOKE COUNTY...
   RAEFORD                5.0  1257 PM  1/10
   ASHLEY HEIGHTS         3.0   103 PM  1/10

...JOHNSTON COUNTY...
   3 NW NEWTON GROVE      1.0   450 PM  1/10
   6 S CLAYTON            0.2   445 PM  1/10

...LEE COUNTY...
   SANFORD                1.0   528 PM  1/10  SANFORD FIRE DEPT

...MONTGOMERY COUNTY...
   TROY                   3.0   123 PM  1/10

...MOORE COUNTY...
   SOUTHERN PINES         5.0   212 PM  1/10
   ABERDEEN               4.0   841 AM  1/10  MEDIA VIEWER

...RANDOLPH COUNTY...
   2 S ASHEBORO           2.0   926 AM  1/10
   SEAGROVE               2.0   910 AM  1/10
   ASHEBORO               1.5   830 AM  1/10
   7 SW ASHEBORO          0.5   610 AM  1/10
   1 W SEAGROVE           0.5   555 AM  1/10

...RICHMOND COUNTY...
   ROCKINGHAM             5.0   911 AM  1/10

...SAMPSON COUNTY...
   CLINTON                3.0   535 PM  1/10

...SCOTLAND COUNTY...
   5 SE LAURINBURG        7.0   311 PM  1/10
   LAURINBURG             6.0   101 PM  1/10  TEMPERATURE 28 DEGREES

...STANLY COUNTY...
   STANFIELD              4.0   157 PM  1/10
   ALBEMARLE              1.0   525 AM  1/10
   OAKBORO                1.0   445 AM  1/10  ROADS SNOW COVERED

...WAKE COUNTY...
   GARNER                 0.1   550 PM  1/10
   5 WSW FALLS LAKE       0.1   445 PM  1/10  VANDEMERE COURT
   1 NW RDU INTERNATION     T   430 PM  1/10  TRACE AT KRDU
   4 SW RALEIGH             T   400 PM  1/10  TRACE IN WEST RALEIGH

Final Forecast Notes: Earlier and Stronger

Latest models, radar, and 850mb flow all point towards increased precipitation amounts here in central NC. There is going to be a gradual decrease in amounts in a line from SW of Charlotte moving NNE as the flow subsides and energy transfers to the coast and forcing weakens.Here is the 6Z NAM output, which prints out almost close to .75" of liquid, and others have been trending similarly.




Snow is already falling in areas to the East and South of Salisbury and I expect snow to begin in earnest this morning (8AM) as compared to previous thought of noon.While the radar looks/will look impressive, it will take some time to moisten the atmosphere as the dewpoint is currently around 0. Once saturated, expect snow to fall most of the day before a late switch to primarily freezing rain/drizzle, which will last into Tues AM.

Final Forecast for Rowan: Generally 5-6" of snow with heavier amounts possible towards the southern and western ends of the county. This will be covered by freezing rain/drizzle ("glaze") overnight into tomorrow, compacting the snow and making conditions more hazardous, especially on roadways.

Sunday, January 9, 2011

NWS Issues Winter Storm Warning

From the NWS in Greenville/Spartanburg:

"... Winter Storm Warning in effect from 6 am Monday to noon EST
Tuesday... 

The National Weather Service in Greenville-Spartanburg has issued
a Winter Storm Warning for heavy snow... which is in effect from 6
am Monday to noon EST Tuesday. The Winter Storm Watch is no longer
in effect.

* Locations... the northern foothills and northwest Piedmont of 
western North Carolina. 

* Hazards... heavy snow... with light accumulations of sleet and 
freezing rain. 

* Timing... snowfall is expected to develop from the southwest
during the morning hours Monday. The snow may mix with... or
change to... sleet in the late afternoon and early evening. A
changeover to light freezing rain is expected Monday evening... 
before gradually ending as a period of light freezing rain and
freezing drizzle early Tuesday.

* Accumulations... snow accumulation of 3 to 5 inches... along 
with a few hundredths of an inch of ice. 

* Impacts... roads will become very slippery quickly on Monday
morning. The combination of heavy snow early in the event... with
light ice accumulations later on... could cause some isolated
power outages.

* Temperatures... highs in the lower 30s with lows in the upper 20s
throughout the winter storm."

Jan 10th Winter Storm Forecast

General Outline = Complex Event.

Expect snow to begin around noon on Monday Jan 10 and continue until approx midnight before changing to sleet briefly and then freezing rain/drizzle overnight and tapering off Tuesday morning.

My Current Forecast for Salisbury/Rowan: 
4" of snow before changeover to freezing rain/sleet mix which will at least put a glaze on everything as surface temperatures stay below 32 until afternoon Tuesday.

While this might not be a monster of a storm, its impact will be felt for a while. Temperatures will remain cold throughout the week, minimizing snow melt and refreezing every night. It should also start during the workday Monday so anyone driving home might be dealing with snow covered roads depending on the exact time of onset.

The trickiest part of the forecast is two-fold:
1. Total precipitation that falls
2. Timing of warmer air moving in aloft - and the corresponding change from snow to sleet then freezing rain.

These two variables will become clearer in the next day. I'll update tomorrow - when you should be out getting your bread and milk. I'll be on the beer aisle.

Saturday, January 8, 2011

BUST

I could write a lot about why my 2" snowfall prediction didn't verify and probably back it up with a lot of big words and fancy pictures (excuses too). Short story: my forecast was a complete bust.

Anyways, Monday's storm has my full attention now. Will have a prelim forecast and maps up later this evening.

Stay tuned

Thursday, January 6, 2011

A Surprise Snow in The Making?

While tomorrow's forecast looks relatively lackluster in terms of snowfall, it seems mother nature might have something up her sleeve. Below is a shot of a high resolution model (WRF/NMM) indicating an area of enhanced (heavier) precipitation tomorrow afternoon/evening just to our east, but in the vicinity...and another short range model, the NAM has followed suit, increasing total liquid (2nd). Their respective 36 hour precip forecasts:

High-Res WRF/NMM

NAM



The NWS in Raleigh has jumped on this and issued Winter Weather advisories for all the surrounding counties to our East and North and I expect ours (Greenville-Spartanburg) to follow suit shortly.

My thoughts: We will be on the western fringe, of the heavier band, so precip will be less and arrive early afternoon, Rain changing to snow around 5 and accumulating approx 2", primarily on grassy/elevated surfaces. Once it starts to rain, the temperature will drop as the column cools, so it's a very tricky scenario. If the precip arrives later, better odds of accumulation and vice-verse. I think a small area to our East will get lucky and find themselves with ~5 inches on the ground. With temps falling into the 20's tomorrow night, roads will freeze if any slush builds up.

If tomorrow is a bust, don't worry - only a prelude to the next few weeks. Stay tuned...

Wednesday, January 5, 2011

A Few Flakes Tonight - Eyes on The Horizon

Tonight we will see rain showers changing to or mixed with snow. I don't foresee anything more than minor accumulation (<1/2") on grassy/raised surfaces - so put your snow shovel up for the moment.


However...next Tuesday brings a much better shot of accumulating snow in the Piedmont of NC. How much - impossible to say at the moment...

Synoptic situation - expecting a trough to move east/south across the plain states and then form a surface low near the GOM that will move NE to a point off the coast of NC. Much like our previous storm but lacking the northern branch feature that really made the low "bomb" as it headed up the coast.

Here are two shots from the GFS. #1 is the 500mb feature and #2 shows the surface feature(s) @ 150 hours. As always, this is almost a week away so it is bound to change.