Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
551 PM CST Wednesday Mar 4 2015
Short term...(this evening through thursday)
issued at 415 PM CST Wednesday Mar 4 2015
Wind chills are the primary area of concern with this short term
forecast. Early afternoon water vapor imagery with 500mb heights and
winds crudely identified a superpositioning of the subtropical and
polar jets across the eastern Continental U.S....with plume of moisture
extending from Central America up through New England coinciding
with an upper level trough. This will be the focus for a significant
storm system expected to produce heavy precipitation across the Ohio River
valley. Closer to home subsidence upstream of the trough axis led to
surface high pressure and mostly clear skies aside from some
afternoon cumulus which were a byproduct of cold temperatures
aloft...and early March sunshine.
Tonight clouds will dissipate after sunset...with winds decoupling
as well. This will set the stage for another cold night...but winds
are expected to be near calm...so do not have a Wind Chill Advisory
issued despite the seasonably cool temperatures. Thursday will see
temperatures rebounding into the teens across most locations as warm
air advection spreads acros the upper Midwest. No measurable
precipitation is expected.
Long term...(thursday night through wednesday)
issued at 415 PM CST Wednesday Mar 4 2015
Weather wise...the extended period still looks fairly quiet...with
only minor precipitation chances expected over the weekend. Still
advertised is the warming to above normal temperatures through
Tuesday...but after Tuesday...models diverge significantly on how
the pattern will evolve.
During the extended...we will be starting with the weather pattern
that has been with US through all of February...ridge west/trough
east. However...through the weekend...we will see the western ridge
break down as a fairly strong closed upper low develops out in the
Pacific between hi and Alaska...with a quasi Rex block developing over
there. This is when the GFS and European model (ecmwf) begin to diverge...with the
the two models ending up about 180 degrees out of phase by the end of
next week...with little confidence in the forecast existing beyond
Tuesday as a result.
For the weekend...as the pattern slowly becomes less amplified and
heights/temperatures finally begin to rebound...we will see a couple
of weak fast moving waves move across the area Friday night and
again Sat night into Sunday. These waves will cause the 850 mb-700 mb temperature
gradient to buckle slight...with resultant warm air advection helping at the very
least lead to periods of increased cloud cover. Whether there is
enough moisture available to create any precipitation is debatable...but with
both waves...models are creating sporadic light amounts of quantitative precipitation forecast...so
allowed for small probability of precipitation the blending procedure to come up with to
remain. Since most of these wave look to come overnight...it should
be cold enough to keep the p-type as snow...but if any sort of
timing changes occurs to where these waves will come in during the
afternoons...then some light rain would be expected in the place of
snow as 925 temperatures warming to a few degrees on either side of
zero...we will easily see temperatures warm into the middle/upper 30s...with
40s possible as well depending on degree of cloud cover.
The real warmth...at least the likes of which we have not seen since
the 2nd through the fourth of November last year comes after Sunday
night...when a warm front will lift NE through the area. In its
wake...those 925 temperatures increase to better than +10c. For now...we
are forecasting highs in the 40s/50s...but the European model (ecmwf) in particular
shows that we could easily see an extra 10 degrees on top of
that...with highs in the 50s/60s more likely. At this point...have
not gone quite that far as there is still some residual snow
cover...so will have to wait and see how clean in terms of cloud
cover those SW winds end up being. Beside highs into at least the
40s...we will see dewps rise into the 30s...so even overnight lows
will be fairly mild...with some lows remaining above freezing
likely...especially Tuesday night.
After Tuesday big model spread gets rather large. The European model (ecmwf) show US
getting under the influence of a Hudson high Wednesday...with
temperatures absolutely off to the raced to end the week. The GFS on
the other hand has another Arctic high coming in on Thursday...with
its warmest day coming ahead of the front on Wednesday...with a
significant cool down coming for the end of the week...with the
threat for accumulating snow to boot. Basically...by Friday...we
have the European model (ecmwf) with 850 mb temperatures pushing +10c with dewps in the 40s
while the GFS has 850 mb temperatures pushing -20c and dewps below zero f. The
discrepancies start Sunday night with a short wave moving across Alaska.
The GFS has a much strong/sharper wave that moves into and across
the northern Continental U.S. By the middle of next week...while the European model (ecmwf)
significantly dampens the wave...allowing the zonal flow and above
normal temperatures to remain in place. Given the presence of the
closed low and blocky pattern out in the Pacific...am a little
sceptical about how strong the GFS is with this wave...but we will
have to wait and see how things play out.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 550 PM CST Wednesday Mar 4 2015
Brisk northwest winds will continue for the next couple hours...with
gusts to 20 knots possible across the region. Winds will begin to
decrease shortly after sunset...with winds going below 10 knots from
around 03z through Thursday morning. VFR conditions should prevail
at all terminals...with a brief exception at rwf as a snow shower
is currently impacting that terminal. Skies will quickly clear
across the area with the loss of instability after sunset...and
remain mostly clear through the rest of the period. Winds will
shift and become southerly around 10 kts Thursday afternoon.
Kmsp...northwest winds gusting to 20 kts will become 10 kts or less after
02-03z this evening. Scattered low clouds are currently clearing
out...with VFR conditions and generally clear skies expected for
the remainder of the period.
/Outlook for kmsp/
Friday...VFR. Winds SW at 10kts.
Sat...VFR. Winds northwest at 10kts.
Sun...VFR. Winds west-southwest at 10g15kts.