North American Mesoscale Model Verification on January 26-27...
The North American Mesoscale Model (NAM) is a high resolution model courtesy of the National Center for Environmental Prediction. Another very common and referenced name for the model is the Weather Research and Forecasting Model (WRF). They are the same model and run out to 84 hours. The purpose of the model is to run a higher resolution determining mesoscale features that often cannot be picked up by the global models due to their larger scale. The NAM can be run on an 80km resolution with a broad view of the United States, or 40km which allows the viewer to zoom in on a localized region to put up on mesoscale features. Also a very high resolution form of the NAM exists using a 12km parametric and can zoom into different states. The model is released four times a day at 0z, 6z, 12z, and 18z. Keep in mind that time is zulu or more commonly known as greenwich mean time.
Computer models are critical to forecasting the weather as they use parametric and other mathematical equations to derive the current and predicted state of the atmosphere using a physical and chemical explanation as the background for the predictions. The NCEP is associated with the National Oceanic Atmospheric Administration (NOAA) to be the developer of these computer models. Each meteorological organization worlwide uses their own developed computer models to serve as a basis for the forecasts. Without computer models, we are not able to derive predictions on the weather greater than 24 hours out.
Like all computer models, the NAM shows all layers of the atmosphere in maps determining positioning and speed of the jet stream, surface precipitation amounts (quantitive precipitation forecast QPF), temperatures throughout the entire core of the atmosphere, convective indices, simulated radars, etc. Due to the higher resolution of the NAM, several problems do exist. It often produces convective elements in many middle latitude cyclones and these are known as convective feedback problems. This is where it developed these unstable regions and often associates them as surface lows causing the surface depiction to be distorted. This was a problem in the recent prediction of the January 26-27 winter storm and will be noted below. Also the NAM has a tendency to produce higher than normal precipitation amounts. I typically find myself cutting totals nearly by 30% as shown on the NAM. Finally the NAM has a bias to overamplify a low pressure and close them off to early particularily on eastern United States cyclogenesis off the coast.
The January 26-27 winter storm featured many model headaches do to some interesting variables. Early on it appeared likely for a low pressure to track up inland along the coastal plain. There was a lacking 50/50 low, unfavorable position of the western trough axis, neutral NAO, natural baroclinicity along the coastline, and stale antecedent cold air mass. This would produce rain along I-95 with heavy snows inland. The 500mb synoptic pattern showed very strong signals for this time of setup with most computer model guidance also in support. The GGEM and ECMWF led the pack with the heavy snows from I-81 on westward. The GFS suffered major problems with varying solutions for each run. But then the computer models began to delay the storm. In fact it was delayed nearly 48-60 hours from the original starting time. This caused a different scenario to unfold. The lacking high pressure to the north was still a problem as the anticyclone zoomed east-northeast, but now a high pressure and associated shortwave out ahead of it allowed sunk east-southeast across the Midwest and western Great Lakes. This acted as a 'kicker' helping to push the cyclonegenesis farther off the coast. This in turn allowed for a colder scenario along with precipitation to occur farther east. Therefore a turn of events allowed for a major I-95 snowstorm with 6in+ totals from Washington DC to Boston. This was very fortunate for snow lovers in that corridor due to pure luck given the poor synoptic setup.
Given the NAM's high resolution, it often overanalyzes prognostics post hour 60. It tends to enhance QPF, overamplify lows, and pick up on mesoscale features that really do not exist. Therefore I typically throw out hours 60-84. It would be a rare event where you would find the NAM 84 hour verifying anywhere close to accurate. But in the near term range, the NAM does an excellent job locating temperature thermals, QPF ranges, and picking up on mesoscale features; coastal fronts, enhanced convection, deformation bands, etc. But in this recent storm, it suffered a plethora of problems and even the 6 and 12 hour surface maps had poor verification especially in the QPF department.
Let us first look at the actual accumulated precipitation totals for the storm.
Given this is a 24 hour accumulated precipitation amount, about .01-.1in of additional precipitation fell south of the Mason-Dixon line in the previous 24 hours.
Here are the preceeding NAM total QPF forecasts...
(January 24; 18z) (January 25; 6z)
(January 25; 12z) (January 25; 18z)
(January 26; 0z) (January 26; 6z)
As you can see, the NAM had a lot of variance with the northwestern sharp precipitation gradients. These tight gradients this year have been caused by the rapid intensification of the coastal lows allowing the heaviest moisture to be confined closer to the center of circulation. Also in this instance, a very cold and dry air mass along with associated cold front was quickly advancing southeast across the Great Lakes and was even picked up on by the 700mb RH charts. This allowed the flow out of the northwest to dry up some moisture for areas more inland.
The NAM simulated radar vs. the forecast QPF did not match up. Often the NAM simulated radar showed the heavier mesoscale bands lining up in northern Maryland and southern Pennsylvania where as it only showed total QPF to be .25in-.5in. In fact looking at total verification, the NAM did very well for its simulated radar.
(Actual NEXRAD National Radar) (6z NAM January 26 Simulated Radar)
The NAM did seem to have a hold on the 500mb map showing the negatively tilted trough producing the coastal low along with the placement of the upper level low and associated shortwave kicker just to the west.
(18z NAM January 24; 500mb) (0z NAM January 26; 500mb)
In general the differences in the 500mb maps were very subtle with just a general strengthening in the closed 500mb low, which verified a tad north of the January 24 18z model run.
The NAM did a very excellent job in identifying mesoscale band using the UUV/700mb RH charts.
(6z NAM January 26; 700mb)
It indentifed the enhanced snow growth over southeastern Pennsylvania up through New Jersey and New York City. The problems with the NAM generally existed in the QPF fields. The model likely suffered a very convective feedback issues in QPF totals. This is why it is important to note other maps than surface maps to help locate the heaviest precipitation. The 700mb map screamed that snow totals would be farther inland with the enhanced deformation band and UUV rates. In general the high resolution models handled this the best with the HRR scoring an amazing victory for QPF along with the ECMWF. The GFS/NAM did a very poor job for QPF, but as noted above QPF does not always tell the story. Sometimes it is important to note other layers of the atmosphere to help make a forecast. This point is why many forecasters missed the boat. Many forecasters (especially broadcoast meteorologists) are drawn to the easy to understand QPF/surface maps, but one has to look at all layers of the atmosphere to make a prediction.
Lower Susquehanna Valley Doppler...
(Courtesy of WGAL)
"Here northeast of Harrisburg 2010-2011 winter statistics"
(Snow Stats)
Current Snow Cover- 0-3in
Monthly Total (November)- Trace
Monthly Total (December)- 0.6in
Monthly Total (January)- 18.90in
Monthly Total (February)- 1.35in
Seasonal Total- 20.85in
Winter Weather Advisories- 7
Winter Storm Warnings- 2
Ice Storm Warnings- 0
Blizzard Warnings- 0
Freezing Rain Advisories- 1
Winter Storm Watches- 2
(Temperature Stats)
Lowest High Temperature- 23.7F
Lowest Low Temperature- -1.7F
Wind Chill Advisories- 0
Wind Chill Warnings- 0
(Snow Storms Stats)
First accumulating snow - December 10 - 0.50in of snow
Clipper light snow - January 7-8 - 2.25in then another 1in of snow
Double Barrel Low - January 11 - 4.5in of snow
Coastal Low - January 17-17 - 1.8in of snow/sleet
Arctic Front - January 20-21 - 2.1in of snow
Upper level/coastal low - January 26 - 5.75in of snow
Two clippers - January 28-29 - 1.5in of snow
Reader Comments
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we're getting a little precip from the back end of the system heading towards all of you.
NWS is sayin 3-7" for Shippensburg, PA and they are under a WSWatch
Hoy - hope you were able to take some pictures as it looked nice on TWC this morning up there.
At the Jersey Shore - I think I caught a glimpse of some new snow on the colder areas of grass - rain for the most part.
There will be a sharp gradient on the north side of the shield tonight so areas near that gradient should keep an eye out for any snow total adjustments. With a low track slightly to the north of forecast and/or any expansion in the shield could go from expecting 0” to expecting 6” in no time. Could also go from expecting 6” to expecting 0” in no time.
THE FOLLOWING ARE 6” PLUS TOTALS FROM LOW #1…
CONNECTICUT:
Greenwich: 7.0”
NEW JERSEY
Newton: 8.0”
Ringwood: 8.0”
West Milford: 7.9”
Butler: 7.8”
Jefferson: 7.7”
Mahwah: 7.6”
Ramsey: 7.3”
Oakland: 7.1”
Allendale: 7.0”
Petersburg: 7.0”
Rivervale: 7.0”
Wantage: 7.0”
Wayne: 7.0”
Marcella: 6.7”
Hawthorne: 6.5”
Ridgewood: 6.3”
Sparta: 6.3”
Blairstown: 6.2”
Fredon: 6.0”
NEW YORK
Frewsburg: 10.3”
Forestville: 10.0”
Wellsville: 10.0”
Whitesville: 10.0”
5 S Corning: 9.6”
Jamestown: 9.0”
Kennedy: 9.0”
Olean: 8.2”
Yulan: 8.0”
Belmont: 7.8”
Elmira: 7.5”
Chemung: 7.2”
Hornell: 7.0”
New Hempstead: 7.0”
Thornwood: 7.0”
Warwick: 6.7”
Painted Post: 6.5”
Scarsdale: 6.2”
Armonk: 6.1”
Harrison: 6.1”
Hornell Almond Dam: 6.1”
Dansville: 6.0”
Hinsdale: 6.0”
Nanuet: 6.0”
Nyack: 6.0”
Windham: 6.0”
Yonkers: 6.0”
PENNSYLVANIA:
Coudersport: 14.0”
Colt Station: 13.0”
Wellsboro: 12.2”
Sabinsville: 12.0”
Westfield: 12.0”
6 SSE Erie: 10.5”
Fairview: 10.5”
Granville Summit: 10.5”
Waterloo: 10.5”
Galeton: 10.4”
Bumpville: 10.2”
Alba: 10.0”
Chandlers Valley: 10.0”
Covington: 10.0”
Moscow: 10.0”
Roseville: 10.0”
Marienville: 9.6”
Laporte: 9.5”
6 SW Northeast: 10.0”
Tobyhanna: 9.3”
6 SW Erie: 9.2”
8 SW Harveys Lake: 9.1”
Corry: 9.0”
Cowanesque Dam: 9.0”
Dallas: 9.0”
2 WSW Erie Airport: 9.0”
Moosic: 9.0”
Pocono Peak Lake: 9.0”
Millcreek: 8.8”
Chapman Lake: 8.0”
Stevenson Dam: 8.0”
West Hickory: 8.0”
Renovo: 7.9”
St Mary’s: 7.9”
Warren: 7.5”
Kane: 7.2”
Amity: 7.1”
Hawley: 7.0”
Lake Harmony: 7.0”
Mountain Top: 7.0”
Ridgway: 7.0”
Wilkes Barre: 7.0”
Blooming Valley: 6.8”
Titusville: 6.8”
Waterford: 6.3”
Franklin Center: 6.1”
Matamoras: 6.7”
North East: 6.6”
Gold Key Lake: 6.5”
Mifflinburg: 6.1”
Beach Lake: 6.0”
Nescopeck Pass: 6.0”
Oil City: 6.0”
Pleasant Mount: 6.0”
Holy Moly, sorry that last post was so long! There were more 6" plus reports than I though and it got out of hand lol.
Cool, guess you have off for President's Day Blizz. How much farther north do you think radar echoes are compared to simulation? I know you guys south of the Mason Dixon don't want this to happen, but I need a 70-mile shift north to place me in the path of highest potential snowfall accumulation. I have Calculus to do and don't have any more time to look at stuff like that.
The timing was right for all snow over this way.
Nope, I had a snow make up for school today. Radar looks about 25-50mi farther north than the 12z NAM. Tonight will be interesting to watch. Latest HRRR model is way north with heaviest snow accumulations from Altoona to Harrisburg to north of Philadelphia.
URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
313 PM EST MON FEB 21 2011
...A FAST MOVING WINTER STORM WILL RETURN SNOW AND SLIPPERY TRAVEL
TO MUCH OF OUR AREA BY MIDNIGHT...
.LOW PRESSURE IN KENTUCKY THIS AFTERNOON WILL SLIDE TO THE EAST OF
NORFOLK VIRGINIA TOMORROW MORNING. ACCUMULATING SNOW OVERNIGHT
WILL GENERALLY END BY SUNRISE TUESDAY. MELTING WILL BE SLOW TOMORROW
AS TEMPERATURES STRUGGLE TO RISE ABOVE FREEZING DURING THE AFTERNOON.
DEZ001-002-MDZ008-012-015-019-020-NJZ016>018-021> 025-PAZ070-071-
221000-
/O.NEW.KPHI.WS.W.0007.110222T0000Z-110222T1400Z/
NEW CASTLE-KENT-CECIL-KENT MD-QUEEN ANNES-TALBOT-CAROLINE-SALEM-
GLOUCESTER-CAMDEN-CUMBERLAND-ATLANTIC-CAPE MAY-
ATLANTIC COASTAL CAPE MAY-COASTAL ATLANTIC-DELAWARE-PHILADELPHIA-
INCLUDING THE CITIES OF...WILMINGTON...DOVER...ELKTON...
CHESTERTOWN...CENTREVILLE...EASTON...DENTON...PEN NSVILLE...
GLASSBORO...CAMDEN...CHERRY HILL...MILLVILLE...HAMMONTON...
CAPE MAY COURT HOUSE...OCEAN CITY...ATLANTIC CITY...MEDIA...
PHILADELPHIA
313 PM EST MON FEB 21 2011
...WINTER STORM WARNING IN EFFECT FROM 7 PM THIS EVENING TO 9 AM
EST TUESDAY...
THE NATIONAL WEATHER SERVICE IN MOUNT HOLLY HAS ISSUED A WINTER
STORM WARNING FOR SNOW...WHICH IS IN EFFECT FROM 7 PM THIS
EVENING TO 9 AM EST TUESDAY.
* LOCATIONS...THE PHILADELPHIA AREA OF SOUTHEASTERN
PENNSYLVANIA... SOUTHERN NEW JERSEY ALONG AND SOUTHEAST OF THE
ATLANTIC CITY EXPRESSWAY...THE NORTHERN TWO THIRDS OF DELAWARE
AND MUCH OF MARYLAND`S EASTERN SHORE.
* HAZARD TYPES...SNOW. THERE MAY BE A LITTLE SLEET AT THE
BEGINNING BUT IT WONT LAST LONG.
* ACCUMULATIONS...3 TO 5 INCHES OF SNOW.
* TIMING...SPOTTY RAIN AND DRIZZLE WILL QUICKLY CHANGE TO SNOW
THIS EVENING WITH TEMPERATURES FALLING BELOW FREEZING BY LATE
EVENING FROM NORTH TO SOUTH. SNOW SHOULD TAPER TO FLURRIES AND
END BETWEEN 6 AM AND 9 AM TUESDAY.
* SNOW FALL RATES...MAY REACH 1 INCH PER HOUR SOMETIME BETWEEN 11
PM AND 4 AM.
* IMPACTS...SNOW COVERED AND SLIPPERY PAVEMENTS ARE ANTICIPATED TO
DEVELOP FROM NORTH TO SOUTH DURING THE EVENING AND VERY HAZARDOUS
TRAVEL IS EXPECTED FOR SEVERAL HOURS TONIGHT.
* WINDS...NORTHEAST 10 TO 15 MPH WITH GUSTS UP TO 25 MPH.
* TEMPERATURES...WILL FALL INTO THE UPPER 20S FROM NORTH TO SOUTH BY
AROUND MIDNIGHT AND BE DOWN TO 20 TO 25 DEGREES BY DAYBREAK
TUESDAY.
* VISIBILITIES...AT TIMES NEAR OR BELOW ONE HALF MILE OVERNIGHT
IN A PERIOD OF MODERATE TO HEAVY SNOW.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
A WINTER STORM WARNING MEANS SIGNIFICANT AMOUNTS OF SNOW...
SLEET...AND ICE ARE EXPECTED OR OCCURRING. STRONG WINDS ARE ALSO
POSSIBLE. THIS WILL MAKE TRAVEL VERY HAZARDOUS OR IMPOSSIBLE.
It started snowing at 3:25! :)
It was nice indeed, but I wasn't out taking photos this time. You'll get some snow tonight.
In fact, if this thing doesn't start going south soon, we're going to get more than the predictied 'flurries' here. Weird...do these people who write up the forecasts merely watch model trends without factoring in what is actually occurring on radar?
Ok, yeah I see how much farther north that shield is. By my perception, it seems that the shield is almost 100 miles north of where the 12Z NAM projected it at 6 pm. There is a serious battle going between the wedging high pressure and northern extent of precip north of I-80 (north of I-78 closer to Allentown PA) and east of Bradford PA. If you were to just look at the radar, you would think that the Pennsylvania Turnpike and along and just south of I-195 in New Jersey are in a prime position. NWS Mount Holly at the moment still at 1" here overnight. I'm sure (I hope) they're watching it closely.
At 6:25, I'm Cloudy with Virga, 33.1F, 24D, 68%
At 6:00 pm, Mount Pocono was Cloudy, 20F, -2D, 38%.
So you can see how dry the air is in northeast Pennsylvania with that high pressure centered over southeastern Canada and wedging its influence into northeast PA. This future interaction is the key to snow totals in eastern PA and central and northern New Jersey.
Stations reporting snow at 6:00 pm.
NEW CASTLE PA / SNOW / 27F / 25D
BEAVER COUNTY PA / SNOW / 28F / 27D
PITTSBURGH PA / SNOW / 29F / 26D
LATROBE PA / LGT SNOW / 32F / 32D
JOHNSTOWN PA / LGT SNOW / 25F / 21D
ALTOONA PA / LGT SNOW / 25F / 21D
YORK PA / LGT SNOW / 28F / 25D
HARRISBURG PA / FLURRIES / 31F / 17D
PHILADELPHIA PA / FLURRIES / 36F / 22D
MILLVILLE NJ / FLURRIES / 36F / 26D
The low is moving west to east from northeastern Kentucky and is forecast to take a southeastern dive towards northeastern North Carolina. The only thing is that the precip shield extends a lot farther north right now than expected.
I understand that this thing is dipping down to the SE, but with the precip shield extending as far N as it is and the precip already pretty far E, it would seem that our forecast for flurries in NYC is a bit too conservative. It looks like we'd get at least a dusting and at most a couple of inches.
Decided to take the car out of the garage on the chance it's closer to the 7 than the 4. Wouldn't want to have to dig my way out to the street. The tires can do it, but the mazda's low clearance is another matter.
Tonight: Snow and sleet, becoming all snow after 10pm. Low around 28. Northeast wind between 13 and 17 mph. Chance of precipitation is 100%. Total nighttime snow and sleet accumulation of 4 to 6 inches possible.
We're not even in the winter storm warning, but that's wsw criteria for us, I'm sure.
Wrong again Stan!
Nope, it is what it is. What you see is what you get, maybe besides a little mountain lift and the usual higher ratios there.
Drooooooool
the radar is showing the snow all around. this has happened a few times to me this season...the radar in Blizz's blog has the area covered in blue except for a small triangle from harrisburg, to lebanon to lancaster.
Hourlies in Hamilton NJ
Cloudy throughout...
5pm, 34F, 24D, 68%
6pm, 33F, 24D, 68%
7pm, 33F, 23D, 66%
8pm, 32F, 17D, 54%
Area forecast discussion
National Weather Service Mount Holly New Jersey
754 PM EST Monday Feb 21 2011
Synopsis...
low pressure over West Virginia this evening will move out to sea
east of Norfolk Virginia Tuesday morning.
Near term /until 6 am Tuesday morning/...
750 pm: no changes to the warning/advisory config at this time. Sort of waiting
on kmdt klns kthv to lower substantially as drying beneath the oncoming
lift is still dominant and eating up snow aloft. If I see it go to 1/2
MDT snow at kmdt or kthv at 9pm...will adjust amts up at that
time. Otherwise forecast going as constructed late today is working fine.
From The 5:30 PM update...
rrq of probable 250 mb speed maximum of 130 kts over
southern New England this evening coupled with the lfq of the 135 knots Ohio Valley
speed maximum heading for korf tomorrow morning is probably the
primary driver of lift. Surface winds in the warm sector across Kentucky at
5pm to 30 kts lifting 50+sfc dewpoints over ever colder low level
wedge of air pressing southward through New Jersey and PA behind this
mornings snow productive and eastward departing first low center.
The lift with this second low is already realized in PA with 1/2
to 1/2sm MDT to heavy snow from southern central PA through much of north
Ohio.
So while the 500 mb vorticity is moving into confluence between the northern
stream in New England and separate Ohio Valley upper level jet...the
lift should be excellent overnight and suspect 1-2/hour snowfall
rates for 2-4 hours in southern PA/North Maryland.. north del into S New Jersey. Exact placement
defines largest amounts. Will update west-southwest at 6pm for timing.
A substantial overnight travel problem looms as temperatures plunge below
freezing and snow begins with very hazardous travel expected in
the warning and advisory zones.
Gusty north-northeast winds of 25-30 miles per hour will cause some drifting during the
night.
Possible upgrade at 8 PM to a warning may be ahead for the region
in southeast PA SW of phl where am concerned about widespread 6 inch amounts.
temp 27F with dewpt of 9F
Campbelltown, PA
cracked an inch here with heavy snow currently. waiting for the low level jet
STATIONS <=39% RH AT 8PM
Cloudy throughout.......NNE Winds
Harrisburg: 29F / 7D / 39%
Reading PA: 29F / 5D / 36%
Quakertown PA: 28F / 5D / 37%
Blue Bell PA: 31F / 4D / 31%
Willow Grove PA: 30F / 4D / 33D / 33%
Northeast Philadelphia PA: 33F / 11D / 39%
Trenton NJ: 32F / 8D / 36%
Somerville NJ: 30F / 8F / 39%
STATIONS REPORTING PRECIP AT 8PM...NNE Winds
State College PA: Lgt Snow / 19F / 10D / 68%
Lancaster PA: Flurries / 30F / 13 / 45%
Coatesville PA: Lgt Snow / 32F / 16D / 51%
Philadelphia Int'l PA: Flurries / 35F / 16D / 45%
Baltimore MD: Lgt Rain / 33F / 25D / 72%
Washington DC: Lgt Rain / 33F / 25D / 72%
Georgetown DE: Mix / 36F / 32D / 85%
Viewing: 1701 - 1751
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