Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1140 PM CDT Wednesday Aug 20 2014

Short term...(this evening through thursday)
issued at 239 PM CDT Wednesday Aug 20 2014

Heavy rainfall and heat are of concern during the short term
forecast period.

The main changes from the previous forecast were-
1/ to shift the heavy rainfall threat east to include the Twin
Cities metropolitan area 2/ to shift the expected area for thunderstorm
redevelopment on Thursday afternoon south and west to include the
Twin Cities metropolitan area /and locations along the Interstate 94

Low level moisture transport will Blossom across the area this evening
as a warm front lifts into the region. Precipitable water values will
respond accordingly and soar to over 200 percent of normal /2+
inches/ by 06z Thursday. While eastward progression of the convective
activity could initially be slowed due to the blocking affect of
the upper low over the Great Lakes region...eventually the low level
jet will help push the convection eastward toward east central
Minnesota during the overnight hours. Widespread precipitation
totals in the 1-2 inch range are expected. Those totals could
potentially be doubled in a narrow axis along and just north of the
warm front where cells train and regenerate as the low level jet

The Thursday forecast has become more challenging on multiple
levels. What was once was expected to be a dry afternoon for much of
the Minnesota portion of the forecast area is now looking less
optimistic. The 12z model consensus points to afternoon thunderstorm
re-development as far south as the Interstate 94 corridor /including
the Twin Cities metropolitan area/. Have included high chance probability of precipitation for
these areas. If this scenario comes to fruition...the high
temperature in Minneapolis would likely not reach 90 degrees...and
heat indices would not top out in the middle 90s. There is slightly
higher confidence in heat indices from 95 to 100 degrees across
south central Minnesota where clearing is more likely. However...all
of this hinges on where the boundaries lie on Thursday and how far
North/East the middle level cap builds. Will need to monitor
convective/cloud trends and make a game time decision on whether a heat
advisory will be needed.

Long term...(thursday night through wednesday)
issued at 239 PM CDT Wednesday Aug 20 2014

Difficult forecast in the long term as model trends have slowed
the development and movement of the western Continental U.S. Trough.

Conditional thunder threat will continue into Thursday evening as
models continue to indicate some redevelopment along the warm front/
convective outflows from the Wednesday night mesoscale convective system. Will have to
hold onto some chance probability of precipitation for this over mainly the eastern
County Warning Area. Expecting warm middle level temperatures and subsidence behind
short wave to preclude development farther to the west. Severe
potential exists as well...with hail...strong winds possible...if
the activity materializes.

The next significant threat of thunder/heavy rain returns mainly
later Saturday to the southwest and spreading across most of the
area Saturday night into Sunday. Cyclogenesis occurs over Kansas/Dakotas
region into Sunday and the surface low lifts northeast across
western Minnesota. The trough to the west becomes negatively tilted
during this period...and takes most of the energy northeast and is
faster with cold frontal passage Sunday night. The front slows to
the east and becomes nearly stationary through midweek as the
western Continental U.S. Trough lags a bit. This will maintain a southwest
flow aloft over the eastern County Warning Area into early next week. Model
uncertainty remains during this period and will keep the higher
probabilities to the east and south Tuesday and Wednesday. We
should see some cooling to the area...with highs expected to trail back
into the 70s in the wake of mondays cold front. This has been
forecast by some longer term ensemble model forecasts for the last
week or so. This should promote a dry period...with the best
chance of this occurring Monday into Tuesday.


Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1127 PM CDT Wednesday Aug 20 2014

The main concern continues to be timing of the heaviest
rainfall/thunderstorms and rain during the next 6 hours with thunderstorms and rain/rain showers already
developing across far eastern South Dakota and wc Minnesota as of 430z. These
storms were developing on the nose of the low level jet which is prognosticated to
continue overnight and veer more SW with the bulk of the rain showers/thunderstorms and rain
moving from the east early...then more southeast across southern Minnesota through
middle morning. Previous tafs were modified for some timing
differences...but again any heavy rainfall will be short lived
with prevailing visibilities around 3-5 sm with an occasional 1/2 to 2 sm
in +tsra/+shra through 15z. Ceilings will be more of a reflection of
heavier rainfall rates...with some lowering during the typical
morning hours from 10-15z...with ceilings lifting by 15-18z to VFR. More
rain showers/thunderstorms and rain will be possible during the afternoon across wc WI...but
timing and coverage is too low for continuing vcsh/thunderstorms in the vicinity during the
afternoon. Winds will be generally from the southeast/S overnight...then more
S/SW Thursday morning...before becoming light and variable during
the late aftn/evening.


Based on the latest short term model information...the best time
frame for rain showers/thunderstorms and rain to affect kmsp will occur after 8z...with 10-14z
the best time frame of heavy rainfall with IFR conds. A gradual
improvement is expected between 15-18z with VFR conds expected by
the afternoon. Winds will be generally from the east/east-southeast overnight...then
more southeast/S Thursday morning...with a shift to the SW/west in the afternoon
before becoming light and variable in the evening.

/Outlook for kmsp/

Friday...VFR with MVFR/thunderstorms and rain late. Winds NE at 10 kts.
Sat...MVFR with IFR/+tsra possible. Winds east at 10g15 kts.
Sun...VFR with MVFR/thunderstorms and rain possible. Winds northwest 10 kts.


Mpx watches/warnings/advisories...


long term...dwe

National Weather Service Glossary of Abbreviations