Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
735 PM CDT Thursday Jul 31 2014
Short term...(this evening through friday)
issued at 358 PM CDT Thursday Jul 31 2014
Surface analysis shows extensive high pressure over much of the
north-central states...that will push southeast into the Ohio
Valley this period. There is a weak area of low pressure over
southeastern Ontario with a cold front extending westward near
the international border. Water vapor imagery continues to nicely
show the persistent upper level low over eastern Canada with
several shortwaves rotating around the western and southern
sides...along with solid northwest flow over the north-central
states. Kmpx radar indicates another afternoon of isolated
showers/thunderstorms over western Wisconsin...with the cells
again driving to the southeast under the mean northwest flow
aloft. The lack of a strong shortwave on the western side of the
upper trough means even lesser storm coverage and intensity
compared to yesterday...but enough such that 20-30 probability of precipitation are
required for the rest of this afternoon into this evening. Another
tranquil overnight period tonight is expected followed by another
afternoon of scattered showers and thunderstorms for western
Wisconsin into eastern Minnesota for Friday. The coverage looks to
be better Friday due to a more pronounced shortwave trough axis on
the west side of the large trough dropping south through MN/WI.
Adding in modest instability and there are more ingredients
available for greater coverage and intensity of storms for Friday
afternoon. Not looking for any organized severe weather but an
isolated strong to severe storm containing large hail and/or
strong wind gusts cannot be ruled out. As for temperatures...more
of the same. Highs on Friday look to be nearly identical to today
for the entire coverage area...with highs hitting the upper 70s to
lower 80s. Low temperatures tonight look to be slightly
higher...owing to a more southerly component in the low level
winds...albeit quite light. Minimum temperatures look to range
from the middle 50s to lower 60s.
Long term...(friday night through thursday)
issued at 358 PM CDT Thursday Jul 31 2014
For the extended...we are still looking for a wetter system to
impact the area in the Tuesday/Wednesday time frame as we
deamplify the pattern that has been with US all of this work.
Although more widespread precipitation will be a change from this
week...the one thing that will not be changing much will be our
temperatures...which will continue to run slightly below to near
The long term period will start with what will be the final
shortwave to rotate around the western periphery of the eastern noam
trough...moving southeast of the mpx area Friday evening. Subsidence and
building heights in the wake of this wave will lead to yet another
clear night Friday night...followed by a dry day Saturday that will
also be a few degrees warmer than what we have seen this week.
Although we will be dry on Saturday...a weak cold front will be
dipping into northern Minnesota...that will be into central Minnesota by Saturday
morning. From Saturday morning through Tuesday...this front will
more or less stall out between central Minnesota and northern Iowa...which means
from Sunday through Tuesday...there are basically chance probability of precipitation every
day as this front loiters in the region. Forcing each day does not
look all that impressive...mainly consisting of weak waves rotating
around the NE periphery of The Rockies ridge. Each wave will lead to
a slight increase in srly low levels winds and associated isentropic
lift over the front...increasing chances for precipitation. However...there
is quite a bit of spread in the models with timing of these subtle
features...hence the fairly widespread smearing of low chance probability of precipitation
across the area from Sunday through Monday night.
The signal for more widespread precipitation begins to increase Tuesday and
looks best Tuesday night into Wednesday morning....where probability of precipitation were
increased into the likely category. During the day on Tuesday...an
upper low that will originate in the SW monsoon this weekend will
begin emerging out of the northern rockies. As this happens...a Lee side
trough will begin to develop from western sodak down through eastern
Colorado and will move along the front stalled out across the plains
Tuesday and Wednesday. A strengthening low level jet ahead of this trough will
result in increased isentropic lift across the nearby
boundary...with afternoon storm coverage expected to increase as a
result. Both the GFS/European model (ecmwf) are now showing a good slow moving mesoscale convective system
signal across southern Minnesota Tuesday night as differential divergence
becomes maximized within the right entrance region of an upper jet
streak that will coincide with a 30-40 knots low level jet nosing up into southern Minnesota.
This has been a very consistent signal in the European model (ecmwf) the last several
runs...hence increasing probability of precipitation up into the likely category Tuesday
night. Both the GFS and European model (ecmwf) show a nice 2-4 inch quantitative precipitation forecast swath working
across The Heart of the mpx County Warning Area and working across central WI. We
should more or less get on the back side of this system Wednesday
night...with the end of next work week looking to be dry.
Though no widespread severe weather is expected...being on the
periphery of the upper ridge...we will be maintaining 500 mb winds
around 30kts...which will keep bulk shear up around 30 kts as
well...so some isolated severe storms can not be ruled out any day
from Sunday through Wednesday. The best potential for severe
weather would be Tuesday night given the potential for an
mesoscale convective system...though the heavy rain signal looks better than the severe
risk. Fortunately...we have been dry the last 3 or 4 weeks...so the
flash flood risk will be no where near as high as what we were
dealing with in June.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 735 PM CDT Thursday Jul 31 2014
Isolated storms in western wisc are weakening. One or two may hang
in there through sunset but should not persist very long
thereafter. Otherwise mostly clear skies with light winds
overnight. Surface winds should mostly be west to northwest at less than 5
knots... with ridge still mainly from western Iowa to eastern South Dakota.
More cumulus expected to develop late morning to early afternoon
onfriday...and isolated showers and thunderstorms will be
possible. Have showers in the forecast but have not added thunder
due to uncertainty of location tomorrow afternoon.
Kmsp...VFR with no significant concerns through 18z. Then more cumulus
will develop. Main question is whether thunder will develop around
kmsp or if it will be confined farther east or south. Have kept
out mention of thunder due to said uncertainty.
/Outlook for kmsp/
Sat...VFR. Winds variable 5 kts.
Sun...VFR. Slight chance thunderstorms and rain. Winds SW 5-10 kts.
Monday...VFR. Chance thunderstorms and rain. Winds SW 5-10 kts.