North American Mesoscale Model Verification on January 26-27...

By: Zachary Labe , 10:34 PM GMT on January 29, 2011

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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

Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011 (Blizzard92)
Melting begins...
Ice Storm 2011
Ice Storm 2011 (Blizzard92)
Melting begins...
Ice Storm 2011

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Blizz, what would you recommend as the best way to monitor the development of the coastal low?

I've been checking pressure readings from weather stations on long island down to VA trying to get an idea of where it might form, but I feel like a blind squirrel. any maps, like water vapor, that there might be some clues on?

seems the lowest pressures on the coast are in the VA beach / Delaware area as of this post.
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6 inches of snow up here in Sutton. Temp is 16 winds North NE and bar is falling. What are your thoughts up this way Blizz, 5 mi outside of worcester, ma?
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temp 29.8f
dewpt 28.0F
winds out of East, very light
barometer 1020.7mb and falling

I was out driving and noticed a lot of the lines having icicles on them. most roads were fine and not much traffic
Member Since: January 26, 2007 Posts: 72 Comments: 6255
Quoting Blizzard92:
Cold air is certainly hanging tough in the Lower Susquehanna Valley. 27.7F here despite the max diurnal heating of the day. Most of the freezing rain if not all of it will be occuring at night, which certainly will not help the situation.


Still 28 here as well in Cranford, NJ. Misty nastiness falling all day, gets harder at times, but my car seems to be glazed over pretty good. Im planning to stay home tomorrow....."ice day"
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The storm coming up the valley now just seems too large, too warm with large warm entrainments, and too far to the northwest to give the Atlantic seaboard anything but rain.

Of course, this offshore low one hears about, or anything that will bring the cold front farther southeast could change it all drastically. Very hard to call for NYC.
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I could be wrong, just from memory. Hopefully the temp goes up a little more, I'll take rain over ice.
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Blizz, on radar anyway, the center of the main storm seems alittle west of where the models depicted it a day or so ago.
Member Since: January 31, 2009 Posts: 0 Comments: 7454
Quoting onoweather:
During the ice storm of 2007 the temp actually dropped for a several hour period as the heavier precip moved in. I think it dropped from 27 to 23 at my house in a few hours.

I checked a few temperature logs for that event and found the temperature to range from 27-30F during the whole event. I could have missed something though.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
During the ice storm of 2007 the temp actually dropped for a several hour period as the heavier precip moved in. I think it dropped from 27 to 23 at my house in a few hours.
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Cold air is certainly hanging tough in the Lower Susquehanna Valley. 27.7F here despite the max diurnal heating of the day. Most of the freezing rain if not all of it will be occuring at night, which certainly will not help the situation.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
First, nice to see P451 and his input. Here in Stamford, just very light freezing drizzle, and Blizz here too my temp. has fallen from 29F 2 hours ago, to 25.6F now, we must be feeling the effects of that weak coastal low you mentioned. Baro. at 30.33F. LT Stamford CT.
Member Since: January 31, 2009 Posts: 0 Comments: 7454
556. bwi
DC area mostly above freezing for now, and for once I'm very grateful.

Nearby Airports
City: Temp.: Conditions: Updated:
» Washington 35 °F Overcast 12:52 PM EST
Andrews AFB 32 °F Overcast 12:55 PM EST
College Park 36 °F Overcast 1:30 PM EST
Fort Belvoir 34 °F Overcast 12:55 PM EST
Member Since: September 4, 2007 Posts: 0 Comments: 1381
Quoting GWphilly:
Interesting that it is still 31 degrees in Philly and the southern NJ Shore (Avalon) has dropped 3 degrees to 37 in the last two hours. Could we actually see colder air then what has been forecast? Is there a chance we see different precip type?

Very weak surface low has shown signs of developing off the coast allowing the winds to shift out of the northeast along the coast therefore bringing a drop in the temperature.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
Interesting that it is still 31 degrees in Philly and the southern NJ Shore (Avalon) has dropped 3 degrees to 37 in the last two hours. Could we actually see colder air then what has been forecast? Is there a chance we see different precip type?
Member Since: February 6, 2010 Posts: 0 Comments: 24
551. bwi
Quoting P451:
All NOAA sites are reporting problems...likely due to high volume. They know it's happening and are trying to correct it.



I sort of wish they hadn't linked to HPC off the main weather.gov homepage. Usually only weather dorks go there, but now it's all gummed up!
Member Since: September 4, 2007 Posts: 0 Comments: 1381
Quoting wxgeek723:
I guess Mt Holly doesn't believe in Freezing Rain Advisories or Ice Storm Warnings? They're really procrastinating on issuing any...

No need for any winter storm warning. Tonights primary precipitation type will be freezing rain for areas north of the Mason-Dixon line and south of I-80. I cannot rule out some sleet, but that will probably be more towards I-80 or a heavier burst of precipitation.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
I guess Mt Holly doesn't believe in Freezing Rain Advisories or Ice Storm Warnings? They're really procrastinating on issuing any...
Member Since: August 28, 2008 Posts: 79 Comments: 3493
Freezing rain has picked up here. It's like an ice rink.

Brooklyn, NY
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It is getting pretty icy out there. I was just out shoveling the drive way and all the precep is a coat of ice on surfaces.

Boy I really hope they cancel our class tonight.
Member Since: May 28, 2008 Posts: 38 Comments: 5303
Quoting P451:
All NOAA sites are reporting problems...likely due to high volume. They know it's happening and are trying to correct it.


They need to get it fixed. This is a historic blizzard going to occur in the Midwest including some major cities along with significant iciing from Indianapolis to central Pennsylvania.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
Our forcast high temp was 27 today, we are at 15 degrees in Sutton and 18 degrees 25 mi south in Smithfield. I really hope we end up with snow with this next round or I am in some trouble!
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543. bwi
NWS websites are really off. Getting hazards for tennessee and mount holly off the LWX page in instead of MD/DC/VA. Warning graphics look wrong too on the county map. Bizarre. I hope wunderground has it right, because that's the only reliable way to get information at this point!
Member Since: September 4, 2007 Posts: 0 Comments: 1381
pretty decent rain shower just moving through here. Temp is 28 degrees, from my office window it appears that cars are glazing over, my window of my car looks thick.

Cranford, NJ
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They downgraded our WSW to a WWA. That's more like it.
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Quoting breald:
The 2nd half of this storm is suppose to be out of here by noon tomorrow with mostly rain. I guess the storm is trending warmer than anticipated? I can live with this.


Sounds good to me.
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The 2nd half of this storm is suppose to be out of here by noon tomorrow with mostly rain. I guess the storm is trending warmer than anticipated? I can live with this.
Member Since: May 28, 2008 Posts: 38 Comments: 5303
Temperature was around 29.5 most of the morning but has now dropped to 28.7. Interesting. The snow is totally encased in ice, so much so that you can walk atop all 18" of it. My igloo walls are stronger than they'll ever be, haha.
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537. bwi
DC up to 34 with calm winds. They put up a freezing rain advisory for tonight from 9pm to 7am, which I think means they think temps will fall after sunset. I sort of doubt it myself -- I think we'll stay above freezing at least inside the beltway. Maybe some patchy freezing rain in cold spots. Just my two cents -- we'll see.
Member Since: September 4, 2007 Posts: 0 Comments: 1381
Quoting TheRasberryPatch:


what are you expecting from this storm? rain to move in by late afternoon? then rain with below freezing temps all night into early tomorrow morning? then falling temps and cold the rest of the week?

Yep, you got it right. If we do not get above 30F, then I think a pretty significant ice storm is possible. Although the storm in the west is already stronger than models indicated. Not sure if that will draw up more warm air from the south or not. Currently my winds are out of the east.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
Yep, I think even in the city this will be a pretty serious ice storm. You guys 50 to 150 miles further inland are going to be encased in it.
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534. RkTec
Emmaus, PA

25 degrees
Freezing drizzle
0.1" of sleet fell overnight
0.15" of ice now on top of sleet from freezing rain

Ended up getting no snow so far with this storm.
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Quoting Hoynieva:
We've had freezing mist since I woke up 4 hours ago. This will continue until the heavier precip gets here late tonight, so I'm thinking we'll have quite a lot of ice on cold surfaces by the morning. Plain rain after that. But tomorrow morning's commute in NYC might not be affected too much as there's more than enough salt on the roads.


We are starting to have freezing rain here, too.
Member Since: May 28, 2008 Posts: 38 Comments: 5303
We've had freezing mist since I woke up 4 hours ago. This will continue until the heavier precip gets here late tonight, so I'm thinking we'll have quite a lot of ice on cold surfaces by the morning. Plain rain after that. But tomorrow morning's commute in NYC might not be affected too much as there's more than enough salt on the roads.
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Quoting Blizzard92:

Since this is a two part system, perhaps they are waiting to issue the warning until you get closer to the second wave of precipitation towards early tomorrow morning.


what are you expecting from this storm? rain to move in by late afternoon? then rain with below freezing temps all night into early tomorrow morning? then falling temps and cold the rest of the week?
Member Since: January 26, 2007 Posts: 72 Comments: 6255
Quoting Blizzard92:

Since this is a two part system, perhaps they are waiting to issue the warning until you get closer to the second wave of precipitation towards early tomorrow morning.


It's possible. Ice storm warning for parts of NY and Ct. As tired as I am of snow, I'll take the snow over freezing rain any day.
Stay safe everyone.
Member Since: May 28, 2008 Posts: 38 Comments: 5303
Quoting Blizzard92:

Probably only northwest New Jersey for the severe iciing.


I am like 5 miles east of 287 at 500 ft. I feel like I am right on the border, no?
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Quoting breald:
Can someone who understands the way NWS does things explain this to me. My area is suppose to receive 3-5 inches today, an inch od snow sleet and 1/10 of an inch of freezing rain tonight, and 6-10 inches Wednesday, but not warning. I keep on checking to see if they will post something. Any thoughts as to why?

Since this is a two part system, perhaps they are waiting to issue the warning until you get closer to the second wave of precipitation towards early tomorrow morning.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
This is ugly. Look out in the triangle from Harrisburg to Lewistown to Altoona tonight.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
Can someone who understands the way NWS does things explain this to me. My area is suppose to receive 3-5 inches today, an inch od snow sleet and 1/10 of an inch of freezing rain tonight, and 6-10 inches Wednesday, but not warning. I keep on checking to see if they will post something. Any thoughts as to why?
Member Since: May 28, 2008 Posts: 38 Comments: 5303
Quoting LettyS:
NWS just changed our WSW to an Ice Storm Warning:

... Ice Storm Warning in effect until 6 PM EST Wednesday...
... Winter Storm Warning is cancelled...

The National Weather Service in Upton has issued an Ice Storm
Warning... which replaces the Winter Storm Warning that was in
effect.

* Locations... interior portions of the lower Hudson Valley and
northeast New Jersey.

* Hazards... freezing rain and sleet.

* Accumulations... a 1/4 to 1/2 an inch of ice accumulation... by
Wednesday afternoon.

* Timing... snow has tapered off... and occasional light freezing
rain and freezing drizzle will fall this afternoon through this
evening. Widespread freezing rain and sleet will overspread the
region after midnight tonight and will continue through late
Wednesday morning. Freezing rain and sleet will taper off
through Wednesday afternoon.

Good call. I think several NWS offices should do this and changing from winter storm warning to ice storm warning for areas expecting only freezing rain tonight.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
NWS just changed our WSW to an Ice Storm Warning:

... Ice Storm Warning in effect until 6 PM EST Wednesday...
... Winter Storm Warning is cancelled...

The National Weather Service in Upton has issued an Ice Storm
Warning... which replaces the Winter Storm Warning that was in
effect.

* Locations... interior portions of the lower Hudson Valley and
northeast New Jersey.

* Hazards... freezing rain and sleet.

* Accumulations... a 1/4 to 1/2 an inch of ice accumulation... by
Wednesday afternoon.

* Timing... snow has tapered off... and occasional light freezing
rain and freezing drizzle will fall this afternoon through this
evening. Widespread freezing rain and sleet will overspread the
region after midnight tonight and will continue through late
Wednesday morning. Freezing rain and sleet will taper off
through Wednesday afternoon.
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Quoting danielb1023:


Hey Blizz, could you tell me where in New Jersey you expect there to be serious icing issues tomorrow? Debating about planning a "snow day"

Probably only northwest New Jersey for the severe iciing.
Member Since: December 14, 2007 Posts: 282 Comments: 15100
Quoting Blizzard92:

You won't get any more snow probably. I never understand why Upton had that much snow forecast. There will be some freezing rain tonight, but I think you change to plain rain relatively quickly. New York City is not a good location for ice storm formation.


Hey Blizz, could you tell me where in New Jersey you expect there to be serious icing issues tomorrow? Debating about planning a "snow day"
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Still snowing moderately as it has been since about 7 when i woke up....hard to tell accumulations and they've been plowing the sidewalks and roads.

Also noticed how forecast totals have dropped across the board from chicago to areas in the northeast. I'm guessing something must have changed?


Springfield, MA
Member Since: February 23, 2010 Posts: 0 Comments: 676
I hope no one in the middle of the country had plans today...looks like a freakin alien monster

Edit: this was pulled from the NWS full loop radar but its basically useless. Might load one frame if that right now. So overwhelmed by traffic I'd imagine
Member Since: August 29, 2006 Posts: 0 Comments: 169
517. 900MB
Quoting Blizzard92:

You won't get any more snow probably. I never understand why Upton had that much snow forecast. There will be some freezing rain tonight, but I think you change to plain rain relatively quickly. New York City is not a good location for ice storm formation.


You can't win em all! We've been snow hogs here! Speaking of which, are we getting our snow back on Saturday?
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Cornell University- Atmospheric Sciences Undergrad; Research Assist.- Onset of Spring Indices Toolbox; Interests- Small spatial scale climatolology

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Linglestown, PA
Elevation: 520 ft
Temperature: 24.2 °F
Dew Point: 15.2 °F
Humidity: 68%
Wind: Calm
Wind Gust: 9.0 mph
Updated: 10:37 AM EST on January 18, 2014

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