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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depends on if it's an inch of freezing rain or an inch of sleet. An inch of freezing rain is huge. I think only New London and points east change to plain rain.
Well that is me.
that is amazing because northern Baltimore suburbs were in the mid teens.
I dont know Breald, you could see 3-6 inches there before icing. Suprised you dont have a watch yet. Wouldnt be suprised if one pops up there soon.
National Weather Service Chicago/Romeoville Illinois
638 am CST Monday Jan 31 2011
Discussion...
short term (monday through wednesday)....
The headliner of winter events doesnt arrive until Tuesday
afternoon...however first we get to deal with opening act
tonight...
Lead shortwave in the southwesterly flow aloft is seen on water
vapor imagery early this morning moving western Kansas/Panhandle region.
As this shortwave rides SW flow into the region tonight look for
snow to break out as it begins to tap into better moisture supply as
a result of 30-40kt low level jet. Broad region of isentropic ascent
and lack of well defined focus makes quantitative precipitation forecast forecast tonight
tricky...however models do seem to be keying in on zone of
strengthening low level frontogenesis over southern County Warning Area for somewhat
higher quantitative precipitation forecast amounts. Southern County Warning Area will also have warmer temperature profile
and with models frequently underestimating magnitude of warming in
strong warm air advection regimes have opted to introduce chance of sleet with the
snow over far southern County Warning Area tonight.
Warmer thermal profile and potential for sleet both argue for lower
snow:liquid ratios south where higher quantitative precipitation forecast occurs. While lower quantitative precipitation forecast
and higher slr's farther north support similar snowfall totals and
generally sticking with a 1-3 inch forecast tonight...though local 4
inch amounts wouldnt be at all surprising. Lead shortwave moves east
of the area by early Tuesday morning with snow tapering off/ending
and possibly even transitioning to a period of freezing drizzle
Tuesday morning into the early afternoon.
Hopefully any frdz will be insignificant enough to provide Road
crews a chance to catch a few hours of sleep early Tuesday before
the big hullabaloo starts Tuesday. Overall agreement in models on
synoptic details associated with this system are excellent...with
just the 00z NAM a little weaker/faster with the system and largely
being discounted.
I cannot stress this point enough:
model accuracy at this distance is not such that its wise to offer
any degree of certainty with respect to snowfall totals. Strong
synoptic systems...such as this one...where middle-level dry intrusions
typically result in very tight gradients in snowfall totals along
the south edge of the heavy snow band. Even with very strong model
agreement...it is not uncommon to see forecast storm tracks in models
shift by 100 miles or more in as little as 24 hours from the arrival
of the storm with rapidly deepening/occluding lows. Such a shift in
storm track could result in areas forecast to get 1 to 2 feet of
snow ending up dry slotted with drastically lighter accumulations.
Given the lingering uncertainty...no plans to make upgrade any of
the watch at this time. Having said all this...here are the
highlights of the storm as they appear now...
Snowfall:
intense 60kt low level jet will be very efficient in transporting
Gulf moisture northward...supplying the cyclone with ample moisture
both within the warm conveyor belt and within the developing
trowal/deformation zone. Quantitative precipitation forecast in reasonably good agreement among the
various models and using a reasonably conservative 10-12:1 slr
results in snowfall totals of 8-14 inches Tuesday night alone.
Probably Worth noting that calculating/measuring the actual
snow:liquid ration that occurs may well be difficult or nearly
impossible given the extent of blowing/drifting expected. In
addition...very strong winds could rip dendrites apart...further
lowering the effective snow:liquid ratio.
Some additional accumulating snow is likely Wednesday morning...with
heaviest totals downwind of Lake Michigan in NE Illinois as improving
thermal profiles result in increasingly favorably lake effect set
up. Lake effect should shift east into Northwest Indiana during the
afternoon Wednesday with strengthening cold air advection resulting
in intensifying lake effect snow Showers. Lake effect alone could
result in several more inches of accumulation in NE Illinois and probably
more than that in Northwest Indiana. As mentioned earlier there will be a
sharp north-south snowfall gradient and with GFS/WRF-NAM both
suggesting dry intrusion making it into southern County Warning Area late Tuesday
night...confidence in totals over a foot County Warning Area-wide is moderate.
Wind:
deepening low should result in strong pressure gradient Tuesday
night...with strong winds continuing into Wednesday as strong
isallobaric component kicks in with quick departure and filling of
the low. Forecast soundings from 00z guidance suggests winds will
increase to 25-35 miles per hour with gusts of 40 to possibly 50 miles per hour common.
However...near the lake winds will be even stronger...probably
30-40 miles per hour sustained with gusts of 50-60 miles per hour possible.
Blizzard conditions:
very high quantitative precipitation forecast totals raise doubts about the blow-ability of the snow
that falls. However snowfall rates of 2-3 inches per hour appear
likely for a time Tuesday night which will probably be enough to
knock visibilities down to less than 1/4sm satisfying the Blizzard
Warning criteria. Further increasing the threat of near zero
visibilities is the potential for thundersnow. Will be a very
dynamic storm system with intense upward vertical velocities as
intense frontogenesis works in concert with tremendous upward motion
resulting from fast moving...rapidly deepening/closing off middle level
circulation becoming negatively tilted as it approaches from the
south. Forecast soundings suggest that 700-500mb lapse rates within
the dry intrusion will increase to between 7 and 8c/km...with some of
that instability likely getting tapped into and probably resulting
in embedded thunderstorms.
Lakeshore flooding:
northeast winds prognosticated to increase to 30-40 miles per hour with gusts of 50-60
miles per hour makes Lakeshore flooding a big concern. High resolution modis
visible satellite imagery from Jan 28th showed that ice that had
accumulated near the Illinois shore had been blown well offshore...and
appeared somewhat fragmented in nature east of the Illinois shore.
There is some uncertainty with respect to how much that ice will
retard wave development near the shore...but with 14-18ft
significant waves and occasional waves over 25ft offshore
potentially taking a toll on ice have opted to err on the side of
caution and hoist a Lakeshore Flood Watch. Its possible that ice
could mitigate the coastal flood potential...but hard to say for
sure and if ice isnt a big deterrent then the coastal flooding could
be very significant.
Izzi
Not liking the trend toward colder temps...
Need it to change to rain here for wed afternoon.
Have I mentioned I dont like you this week!?!
Hoy - just curious did you look into moving your flight out up a day? i did that in the Dec. 2007 storm and was so glad I did. just something to consider, even if it is for a few bucks.
TRP, I wish I could do that, but this is pretty much a turbo trip I'm doing and our rainforest zip line/jungle trek and snorkeling outing have been booked. At most I could be delayed 18 hours and still make it in time. Fortunately it's a direct flight so I won't have to worry about delays in any other airport.
And as for moving it to tomorrow, well, that would cost too much. If they were offering, I'd definitely do it, but they're not.
My forecast is pretty darned similar to the last storm- snow, ice, sleet, rain, and then snow for NYC with accumulations of 2"-5" and up to a half inch of ice. Last time we had this forecast we ended up with 19" in the Park!
Can I believe that it will really warm to 38 degrees on Weds? Thoughts?
I agree. This obviously not going to be that big of a deal for most of Southern New England and the coastal Mid Atlantic states.
I guess it all comes down to where the low sets up and I am getting mixed signals. Some see a low over Eastern PA, and with that I understand our warm up and rain, but others are pointing to off the Delmarva or off Jersey. Off Jersey I can see how we'd get a might mix, but a little further out and I think we are in the same boat as the last storm. So, I guess the consensus has the low too close or West of NYC.
But, how can I complain- 56" of snow and counting this season!
It's been quite a year!!! Still think we'll sneak 6-8" before changeover. Looks like 24 hours of snow, sleet, and freezing rain (6am tues-6am weds) before straight rain.
High for Weds now forecast for 35 degrees (was 38 earlier). Much of the storm forecast is for 29 degrees and freezing rain. The only hours above freezing for the next few days is from 2pm-8pm weds (33-35 degrees). Just saying, a slight shift East or slightly colder air and who knows? After all, 48 hours ago we were in the blizzard target.
Now, back to reality....
The trend all winter has been to be colder than what is shown 36-48 hours prior to the event. Not saying that will happen but wouldnt shock me. Once the low forms off the coast I would suspect that temps along the coast will stop rising and begin their descent. Like you said, it could be only about 6 or so hours above freezing although those are likely the 6 hours with the highest precip amounts. Interesting storm...........
36.0 f
sunny and melting nicely
hoping this whole thing goes out and hits the fish
Just huff and puff and blow it up here to Maine! I'm still waiting for my big storm.
Big time slow.
Southern Maine should get a lot of snow. At least a foot, probably closer to two.
WSWatch for MoCo MD up Tues PM through Wed. 1/4" of ice expected. Tonight should be minor w/light sleet/ice, so they are saying.
It's all jammed up!
Whoo hooo, I'm ready!
But I do feel a little guilty wishing this upon the wildlife, so I'll whip up a double batch of peanut butter suet tomorrow for the birds - as a peace offering :)
do you have a link for this map?
I'm not sure if this is the new NAM as i'm still learning the models. 60 hours.
Link
Where do you live in CT again? Somewhere near my house i believe?
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