Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
1054 PM CDT Monday Sep 15 2014
Short term...(this evening through tuesday)
issued at 355 PM CDT Monday Sep 15 2014
A long wave trough axis is just pushing out of the forecast area
as evident by the drying on water vapor this afternoon. We expect
a cool/quiet short term with clearing strato-cumulus deck early this
evening. The main forecast question is how cold to go tonight and
how much frost to carry in the forecast. The previous forecast
mentioned patchy frost in western WI...we expanded the frost in WI
and added patchy frost to the expected cool spots outside the
metropolitan areas in Minnesota late tonight. The 2m temperatures off the NAM tonight
take a good portion of rural areas in Minnesota/WI down into the lower to
middle 30s. This solution is on the cold side of guidance. The
European model (ecmwf) is on the warmer side of guidance...but it has been a bit to
warm /2 to 4 degrees/ on these clear nights the past couple weeks.
We decided to split the difference between the previous forecast
and some bias-corrected model guidance which take more of the
forecast area down into the upper 30s tonight - mainly due to the
drier bl dewpoints being advected in and confidence in clearing
tonight. The limiting factor to frost forming tonight might be the
wind. If we can hang onto a 4-6kt wind late tonight...it should
keep temperatures up at least a couple degrees and keep frost formation
to a minimum...however...with the center of the high just to our
south...we wouldn't be surprised to see a number of locations go
calm or near calm tonight.
Long term...(tuesday night through monday)
issued at 355 PM CDT Monday Sep 15 2014
Large scale pattern evolution...
things look to retain a very familiar cycle of what we/ve seen for
quite some time... with the eastern trough lifting out as the
western ridge builds in but flattens as it does so. After that it
once again appears that we/ll see the western ridge/eastern trough
pattern re-Load by the end of the upcoming weekend.
Forecast item/S/ of concern...
main issue looks to be chances for rain showers/thunderstorms and rain as we get into return
flow while the ridge tries to build into the area during the
latter portion of the week. At this point... things look to be
fairly progressive with no significant widespread precipitation expected.
Locally heavy rainfall could occur with thunderstorms later Friday
into early Saturday. Severe weather does not appear to be a major
concern... although there could be a brief window for a few severe
storms ahead of Friday evening/S cold front with somewhat
favorable instability/shear given expected MLCAPE at or above 1000 j/kg
and 0-6km bulk shear of 35-40 knots.
the deterministic solutions... GFS... European model (ecmwf)... and Gem... are in
fairly good agreement on the overall pattern evolution described
earlier... with the ensemble means showing a bit less
amplification throughout mainly due to differences in timing
amongst the members. The GFS and European model (ecmwf) do have some differences in
their timing and amplitude... but both have bounced around a fair
bit from run to run... so see little reason to prefer either
solution over the other at this point. A blended/consensus
approach seems best with fairly equal weighting of guidance...
which is consistent with the wpc guidance.
Sensible weather expectations...
should see fair and dry weather from tomorrow night through
Thursday with slowly warming temperatures as the ridge inches its
way eastward. More robust warm advection sets up Thursday night
into Friday... which will allow for probability of precipitation to increase from west to
east. Kept some probability of precipitation into Saturday... although it appears the best
chances would be early in the day given an expected frontal passage Friday
night. Allowed for dry weather from later Saturday through
Monday... although there is some possibility that diurnal rain showers
could occur Sunday and Monday depending on the strength of the
upper cold pool. The latest European model (ecmwf) would support that given the
much stronger upper low and cold pool in swings in the region in
comparison to previous runs... but did not include any mention yet
given the inconsistency of the guidance in this regard.
Detailed discussion for item/S/ of concern...
as mentioned... there are not major items of concern... but the
time period when any substantial rainfall and/or severe storms
could occur looks to be Friday afternoon and evening. Instability
is prognosticated to increase with warm advection and moisture return
ahead of a cold front prognosticated to push through Friday night into
early Saturday. Right now the frontal timing is not particularly
favorable with respect to severe weather potential... as both the
GFS and European model (ecmwf) Don/T bring it through until after 06z Saturday.
Should it come a bit sooner there may the possibility of a few
severe storms with marginally supportive instability/shear. With
the system expected to be progressive... chances for sustained
precipitation look fairly limited... so amounts will likely be less than an
inch with the exception of localized higher amounts.
confidence in the overall progression of features and associated
weather is fairly high... this includes the dry period into late
week... chances for precipitation Friday into early Saturday... and cooler
weather by the end of the weekend. The primary uncertainty appears
to be with the timing of things from late Thursday through the
weekend... and subsequent forecasts will likely make some
adjustments and allow for refinement of probability of precipitation into a narrower
window of time. There is also some uncertainty with the
strength/degree of the cold push behind the front over the second
half of the weekend into the beginning of next week. Given what
we/ve seen with this repetitive pattern... one would certainly not
want to rule out another fairly good surge of cold air...
particularly with the European model (ecmwf) showing 850mb temperatures of 0c to
-4c across the forecast area by Monday morning.
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1041 PM CDT Monday Sep 15 2014
High confidence forecast for the next 24 hours. VFR expected to
continue with winds becoming light tonight...then back southwest
and increase to 6-9 kts Tuesday afternoon.
Kmsp...no additional concerns.
/Outlook for kmsp/
Wednesday...VFR. Winds vrb 5 kts.
Thursday...VFR. Winds southeast at 10-15 kts.
Friday...VFR likely. Slight chance MVFR with -shra. Winds S-SW 10-20kts.
WI...frost advisory until 8 am CDT Tuesday for wiz016.