Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
319 PM CDT sun Mar 9 2014
Short term...(this evening through monday)
issued at 319 PM CDT sun Mar 9 2014
The main focus into Monday continues to be on temperatures and how
warm they will be.
Currently...westerly low level flow has brought a downsloping warm
air mass off of The Rockies and into the plains and upper
Mississippi River valley. Temperatures have quickly climbed into
the 40s regionally despite the thick snow pack. Temperatures jump
into the 60s and 70s in eastern Nebraska/South Dakota where the
snow has completely melted. Winds have been fairly strong today
though many of the gusts have been limited due to the Stout low
level inversion that is in place. High level cloud cover will
remain fairly widespread due to the upper level jet stream arching
along the US/Canada border from The Rockies to the East Coast.
This...along with the surface pressure gradient staying tight
overnight...should keep temperatures from dropping much overnight
with near or above freezing for lows.
Low level flow will be will be similar to today for tomorrow with
850mb temperatures staying around +7 to +9c. Low temperatures will
start out much higher tomorrow than they did today...so think that
upper 40s still look attainable for high temperatures based on how
temperatures have done across western Minnesota today.
Long term...(monday night through sunday)
issued at 319 PM CDT sun Mar 9 2014
The focus then shifts to the potential for an accumulating snow on
Tuesday and how fast the precipitation will transition from a
wintry mix to this snow.
A middle level trough just off of the coast of the Pacific northwest
will propagate east across The Rockies on Monday and end up in the
Central Plains on Tuesday. At the surface...a weak cold front will
move through the region Monday night and transition into a
stationary front across central Illinois and northern Missouri as
a surface low forms to the west of this boundary in
Nebraska/Kansas. The low to middle level temperature gradient will be
tightening up from the morning into the afternoon on Tuesday and
will create some strong frontogenesis from the surface on up to
around 600mb per 09.12z NAM/GFS cross sections.
There remains some differences between the 09.12z NAM and
GFS/Gem/ECMWF with this main band of precipitation with the NAM
being the most narrow with it. However...they all are showing this
band moving in early on Tuesday with the heaviest precipitation
falling in the late morning and afternoon on Tuesday.
Based on 09.12z NAM/GFS soundings...it looks like there could be
a brief period early on as the precipitation starts in the morning
where the thermal profile is above freezing and could be
rain...sleet or snow. However...cold air is quickly being advected
in through the morning with the profile dropping below freezing
and the warm layer being relegated to the surface. So...the main
question will be with whether surface temperatures will warm enough
during the day to change the precipitation back over to rain at all. With
the expectation being that the precipitation rates could be rather
high due to the strong frontogenesis...expect that it will not
change over and stay mainly as snow through much of this event.
With quantitative precipitation forecast amounts of around 0.15 to 0... around 1 to 3
inches of snow to fall with the heaviest amounts occurring in
southeast Minnesota/northeast Iowa. Once confidence increases with
where the track of the main frontogenetic band sets up...we may
have to go with some higher amounts and possibly an advisory. The
temperature concerns are high enough to keep snow totals down a
bit for the time being.
Beyond that...the focus GOES back to temperatures with some cooler
air coming in on Wednesday before another brief warming trend for
the end of the week. Fairly low confidence with any precipitation
chances as the next few rounds of systems appear to miss the
region to the north or south.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1254 PM CDT sun Mar 9 2014
Gusty south surface winds will continue into this afternoon in
response to deep area of low pressure over western Ontario. Look
for these winds to subside by sunset...then shifting to the west
by middle Monday morning as a cold front works into the region.
Otherwise...VFR conditions expected through the period.
Although low level wind shear criteria will not exactly get met...plan on
west/northwest winds in the 25-35kt range just above 5ooft above ground level
tonight through Monday morning.