Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1243 PM CDT Tuesday Mar 31 2015
Update...for 18z aviation discussion below
Short term...(today and tonight)
issued at 426 am CDT Tuesday Mar 31 2015
This morning the forecast area finds itself under weak low level
flow and predominately clear skies outside of a batch of middle
clouds slipping southeast across western WI. The result of the
light winds in addition to the little rain that fell yesterday has
led to areas of dense fog this morning in west central WI and
extreme eastern Minnesota. This fog should lift shortly after sunrise.
Cold advection behind the clipper system that passed through
yesterday will keep the eastern forecast area cool through the
noon hour...but warmer 925-850mb temperatures will be pushed into Minnesota this
afternoon. Overall...today will be mild and dry with little wind
after a cool start. Some of the middle level relative humidity trends suggest a
scattered to broken middle deck this afternoon. The level flow will
increase tonight as a surface low slides east across the High
Plains and deepens in ND.
Long term...(wednesday through monday)
issued at 426 am CDT Tuesday Mar 31 2015
Main focus in the extended forecast period continues to be the
potential for the thunderstorms...and perhaps the first severe
weather of the season Wednesday afternoon/evening. The threat has
increased across southern Minnesota...with even better chances to the south
from the Iowa/Minnesota border down toward the NE/Iowa border.
The threat for storms hinges on the low level moisture and
convergence since both are needed to have any shot for surface based
convection. First off...the low level moisture. Initially this
seemed overdone by the models when simply looking at surface
dewpoints. They start off in the 30s Wednesday morning...and quickly
bloom into the low/middle 50s by early afternoon. This seems
unrealistic given the fact that forecast soundings show mixed layer
winds 30-35kts...with 40-45kts atop The Channel. One would think
that this mixing would dry out the boundary layer and quickly erode
the 50f surface dewpoints...but that conceptual model does not apply
to this event.
As is usually the case...the forecast soundings are the key to
determining the origination of the higher dewpoints. Careful
analysis of the southwestern sites...namely kfsd/krwf show a surge
of moisture advection within the h900-800mb layer Wednesday morning.
Precipitable water increases from around 0.55 at 7am to 0.85 by 11am. This
increase is before the model generates precipitation...thus making a more
true measure of the atmospheric moisture. As this elevated moisture
plume advects north/eastward...it mixes down to the surface.
This...combined with daytime heating will destabilized the boundary
layer and result in ml cape profiles of around 1000j/kg across
southern Minnesota by Wednesday afternoon.
This instability will be capped off by a warm layer aloft.
However...upper level height falls will approach from the west late
Wednesday afternoon/evening...inducing ascent and cooling in the middle
levels of the atmosphere. Forecast soundings show cooling
temperatures aloft...and then intitiate convection shortly
In summary...the fact that the 50f surface dewpoints are mixed down
from an elevated moisture plume raises confidence that they come to
fruition. The timing of the Omega/cooling associated with the height
falls is still suspect. If too slow...the threat for surface based
convection will disappear toward sunset. Lastly...the low level
convergence is also suspect...but confidence is increasing that it
will be sufficient to initiate convection. Surface winds off the NAM
back with time across southern Minnesota ahead of the front...likely a
result of the surface trough that develops in eastern NE. Therefore
feel that the threat for strong/severe storms is real Wednesday
afternoon evening as far north as the southern half of MN/WI. The
main threat would likely be wind given the low level speed (not
directional) shear...with a secondary threat of hail. At this time
the tornado potential is low given the expected linear convective
Mode...and uncertainty in surface based convection. The hires 31.00
arw/nmm support this.
Looking ahead...Thursday will be windy with low relative humidity. Northwest flow
will set up with seasonable temperatures through the end of the
week. There is potential for widespread precipitation of around a half an
inch early next week.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1243 PM CDT Tuesday Mar 31 2015
VFR conditions are expected through the period with the main
highlights being increasing middle/high clouds...strengthening
winds...and thunderstorms nearing the area at the end of the
period. The broken deck over western Minnesota will continue to meander
eastward across central Minnesota this afternoon...and western WI this
evening. Otherwise...not much to talk about until tonight when the
winds shift to southeasterly in direction and increase from light
speeds to sustained 15-20kts with gusts to 30 kts as a warm front
lifts into the area. By middle morning Wednesday...sustained speeds
could be approaching 30 kts over southwestern Minnesota /krwf/...with
gusts nearing 40 kts. Thunderstorms are expected to develop on
Wednesday afternoon...with krwf the first site to be potentially
scattered-broken clouds around 9kft move into the site this afternoon...and
then scattered-broken high clouds /20-25kft/ prevail tonight. Wind gusts
into the middle teens are possible this afternoon...then speeds
decrease below 10 kts after 22z and stay there through midnight.
Should see speeds increase toward daybreak and a shift to
southeasterly as the warm front approaches...with speeds
increasing to 20g30kts by middle morning Wednesday. Rain showers/ts chances
increase toward 00z Thursday.
/Outlook for kmsp/
Wednesday night...VFR. Rain showers likely/chance MVFR tsr. Winds S 20g35kts.
Thursday...VFR. Winds west 20-25g35 kts.
Friday...VFR. Winds northwest 10-15 kts.
Sat...VFR. Winds west 10-15 kts.