Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
655 PM CDT Wednesday Mar 12 2014
for 00z aviation discussion below
Short term...(this evening through thursday)
issued at 333 PM CDT Wednesday Mar 12 2014
Surface ridge centered over Minnesota this afternoon...but a sharp warm front
can be seen on its heels from near Minot down to Sioux Falls. Warm air advection
out ahead of the warm front has resulted in a band cloud cover
setting up over western Minnesota that will slowly work east with the front.
These clouds will make it a tough temperature forecast for
tonight...as once they move over a location that will put an end to
the nocturnal temperature plunge. At this point...warmed lows over
Minnesota a few degrees where clouds will have biggest impact and lowered
temperatures a few degrees out in the far eastern County Warning Area...where combo of clear
skies and light winds to start out the night will promote a rapid
drop off in temperatures. In fact...would not be surprised at all if places
like Eau Claire and Ladysmith were able to drop below zero before
the clouds moved in.
On the precipitation end of things with this front...all 12z guidance came
in with two trends for the probability of precipitation...slower and lower. In fact...the
lower trend was low enough that we decided to remove any mentionable
probability of precipitation completely...instead mentioning some flurries as the clouds
work across the County Warning Area tonight. The big problem is it looks like what
little upper support there is from the left exit region of an upper
jet comes in ahead of the isentropic ascent...which looks weaker and
drier than it did yesterday. As a result of this...all
models...including hi-res reflectivity forecasts from the nmm/arw
depict very little if anything falling...hence the flurry approach.
Behind this warm front...winds will turn to the SW...bringing up
another very pleasant and mild Pacific airmass...with 925 mb temperatures
increasing to between +4c and +8c. Current forecast looked to have a
good handle on highs for Monday...so simply blended in the 12z bias
corrected highs from the European model (ecmwf)/Gem /best with verification during
the warmer stretch back on sun and Monday/ with the going forecast.
This resulted in very little change to the going forecast with highs
in the 40s over the snowpack and 50s west of it.
Long term...(thursday night through wednesday)
issued at 333 PM CDT Wednesday Mar 12 2014
The extended period remains uneventful as the main jet core and
associated enhanced weather disturbances remain north or south of the
upper Midwest. Only slight/chance probability of precipitation are noted as two weak frontal
boundaries move across the region Friday...and again late in the
weekend and into early next week. Overall...temperatures will remain blw
average through this period leading to a slow snow melt. The warmest
areas will remain across wc/SW Minnesota where the snow cover is almost
non-existent or very little.
Consistency in both the GFS/ec for next week remains low...so
forecast confidence is also low. The overall jet axis which has been
more northwest/west for the bulk of the winter season needs to change for US
to significantly have a warmup. Some signs of a change is noted late in
the extended period where the jet axis shifts to a more SW/west-southwest
direction. This will lead to better moisture advection...especially
from the Gulf of Mexico. However...at the same time period...the
snowpack may lead to a sharp contrast in temperatures and how the
interaction of the frontal boundary and possibly the low level jet
reflects the precipitation chc/S.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 656 PM CDT Wednesday Mar 12 2014
Narrow ribbon of MVFR to low-end VFR clouds associated with a warm
front will gradully shift eastward across the taf area this
evening. Continued to follow the trends of the rap/sref/NAM
925-850 relative humidity field...which wisp the band of low clouds from west to
east across the Minnesota portion of the area between 00z and 09z
Thursday...and across WI sites between 09z and 16z Thursday. Have included a
broken 2000ft deck during that time frame. Could see a few flurries
as well...but chances for measurable snowfall are displaced to the
north/northeast. South-southwest winds at 5-9 kts will prevail
through Thursday morning...and veer slightly to southwesterly
window of possibility for MVFR level clouds will primarily be between 06z
and 12z...with scattered-broken middle/high clouds and VFR conds on both
sides of that time frame. Southeast winds develop by 04z as the
warm front nears...then prevail into Thursday morning. Southerly
winds prevail after 14z Thursday...then veer to around 210 degrees by
/Outlook for kmsp/
Friday...VFR...chc MVFR ceilings aftn/night. Winds west 10-15 kts.
Sat...VFR. Winds north-NE 5-10 kts.
Sun...VFR. Winds NE-southeast 5 kts.