Transcript 2012_02_23_sneakyNWflow_snowevent.pptx

Beware the NW Flow! Snowstorm/burst
case of 23 Feb 2012 across eastern Colorado
• A well-predicted wave moving southeastward in NW flow aloft
• Wave was in the models at least a week in advance
• But at the surface things far LESS predictable
• A couple of SREF forecasts from 22 Feb are shown
– Not very useful for this case
• Then a comparison of runs from 00z/23 Feb
– The band was shown in the forecasts, sometimes with good position
• But generally too weak and fast-moving
• Big bust was that between 12 and 15-h out the band totally dies in the forecast, while in reality
it cruses across southeast CO with significant snows and winds that were not forecast
– Front position was decent at times
• But wind generally did not stay northerly or NNE long enough or was not strong enough
– Wind returned to NW too quickly behind the front along the Front Range
Snowfall totals from CoCoRAHS
Overview: Reflectivity and obs at 0400z 23 Feb
Front just showing up at the northern part on this scale.
Overview: Reflectivity and obs at 0500z 23 Feb
Front just showing up at the northern part on this scale.
Overview: Reflectivity and obs at 0600z 23 Feb
Part of the front now appears as a nice fine line from KCYS. Notice all the lightning flashes in WY and the well-defined band behind the front.
Overview: Reflectivity and obs at 0700z 23 Feb
The front continues to move south to the east of CYS (note the wind shift at Wray), but the westerlies for now keeping it out of CYS.
Overview: Reflectivity and obs at 0800z 23 Feb
The front finally makes it by CYS and looks to be passing KFNL.
Overview: Reflectivity and obs at 0900z 23 Feb
The front is on the doorstep of BOU-DEN. A brief period of some significant rain (!) hits Boulder just ahead of FROPA.
Overview: Reflectivity and obs at 1000z 23 Feb
An impressive band of reflectivity with snow and honking winds moving through Denver. Note a few more lightning strikes this hour.
Overview: Reflectivity and obs at 1100z 23 Feb
The band of reflectivity is progressive east of Denver but now reflectivity is lingering BOU-Denver. Still one lightning strike this hour.
Overview: Reflectivity and obs at 1200z 23 Feb
The main band of reflectivity is still progressive east of Denver but continues to linger from BOU-Denver.
Overview: Reflectivity and obs at 1300z 23 Feb
The main band is now heading through the Colorado Springs area but echo bands continues to linger over Denver.
Overview: Reflectivity and obs at 1400z 23 Feb
There could be some seeder-feeder stuff going on to help the band linger over parts of Denver. S+ moving through COS area.
Overview: Reflectivity and obs at 1500z 23 Feb
Last of this series. STILL part of the band with hefty reflectivity lingering over west part of Denver. S+ about to hit PUB.
SREF 12-h mean snowfall forecast (and chance >1”) from 15z/22 Feb run for 12-h ending 15z/23 Feb.
Nothing very helpful here, with <20% chance of >1” of snow.
SREF Total snowfall “plumes” from SPC site for the 21z/22 Feb for BJC and sw Denver
BJC
Near Lakewood
Sometimes there is a lot of spread, but in this case (note the scale) no member has >1” forecast, plus the trend of LESS snow for
Lakewood versus BJC is opposite to what happens.
Comparison of runs from
00z/23 Feb. Image is run
NAM
GFS
total precipitation (color scale
same for all images) with
contours of run total snow
(not available for the HRRR)
for the 15-h ending 15z/23
Feb (except for the GFS,
which is 18-h ending 18z).
Interestingly all runs have a
maximum on the plains generally
east of Denver, with the exception
of the FIM (perhaps also the GFS,
but note the minimum in the GFS
forecast along the Front Range).
The NAM has the least
precipitation on the Plains and a
distinct lack of precip up and
down the Front Range.
FIM
HRRR
The high-resolution runs (HRRR/3
km, NMM and ARW at 4.5 km
horizontal grid resolution) as
expected predict more orographic
snow, but only have a slight
indication of a little more snow
over western Denver (and still
<1”).
NMM
ARW
Comparison of runs from
00z/23 Feb – 6-h
forecasts ending 0600z.
NAM
GFS
HRRR
NMM
ARW
Image is observed 0.5o
reflectivity. Model forecasts
are for surface reflectivity
(contours, except 6-h precip
for the GFS) and surface
wind. METARs also
displayed.
The models do have a good
handle on the band of reflectivity,
with the best forecast from the
HRRR. Position of the surface
front and the turn to northerly
winds behind the front are not so
well forecast. Probably the NMM
and ARW show the most northerly
flow behind the front at this point.
NAM
HRRR
NMM
ARW
Comparison of runs from 00z/23 Feb – 9-h forecasts ending 0900z. No GFS for this time.
Once again the HRRR reflectivity forecast most
closely matches the observed band, with the ARW close. None of the models have the wind shift forecast very well, with the NAM still the poorest. The NMM
has more northerlies than the others, but lags well behind the observed front. Winds are turning back to the NW too soon behind the front.
Comparison of runs from
00z/23 Feb – 12-h
forecasts ending 1200z.
NAM
GFS
HRRR
NMM
ARW
Image is observed 0.5o
reflectivity. Model forecasts
are for surface reflectivity
(contours, except 6-h precip
for the GFS) and surface
wind. METARs also
displayed.
In some ways these forecasts are
actually pretty good – they have
the band of reflectivity reasonably
well positioned and have the
leading edge of the front fairly
close to where it is, with a
recovery by the NAM, for the
most part. But closer inspection
shows the reflectivity extent and
strength is under-predicted, the
wind surge too light, and the flow
appears to turn back to the nw
too quickly behind the front.
Comparison of runs from
00z/23 Feb – 15-h
forecasts ending 1500z.
NAM
GFS
HRRR
NMM
ARW
Image is observed 0.5o reflectivity.
Model forecasts are for surface
reflectivity (contours, except 6-h
precip for the GFS, and note that
the GFS forecast is 18-h valid 18z)
and surface wind. METARs also
displayed.
If the forecasts were decent in
terms of the main band at 12-h
our, they are quite poor only 3-h
later. None of the models
maintain the band, which clearly is
still quite strong and brings a
significant and surprising snow to
Pueblo and a lot of Southeastern
CO. Of course also missing is the
echoes lingering across SW
Denver. All the models return to
NW flow right up to the Front
Range north of the Palmer Divide,
which is not observed.
Interestingly the NAM actually has
a decent wind forecast in SE CO.