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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
341 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 corridor/.

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 18z tafs through 18z Thursday afternoon)
issued at 1249 PM CDT Wednesday Aug 20 2014

Few-scattered cumulus/strato cumulus along with some high clouds this
afternoon...then conditions deteriorate significantly tonight into
Thursday morning as a trough enters the region. Leaned toward the
hi-res model consensus for timing of shra/ts...which indicate the
upper low over the Great Lakes region will provide some residual
blocking and hinder the eastward progression of convection
overnight. Expect activity to affect our western sites /kaxn-krwf/
between 00z and 05z...reaching kstc-kmsp-krnh between 05z and
09z. Once the convection develops ceilings/visibilities in the MVFR and IFR
range will become common. The precipitation shield is expected to lift
north and east of most of the taf sites /except possible kstc/
throughout Thursday morning...with some clearing/destabilization
developing over south central into central Minnesota during the
afternoon. Additional development is then expected...but
confidence on the amount of destabilization that will occur and
where the key boundaries will lie is somewhat low at this point.
Winds will be from the east/southeast through the period generally
below 10kts.

few-scattered MVFR to low-end VFR clouds this afternoon...then rain showers/ts
arrive around/after 05z. The main time frame for heavy rain showers/ts
appears to lie between 09z and 14z...with lighter/scattered activity
expected for a couple hours on both sides of that time frame. A
break is expected late Thursday morning and early afternoon...with
additional ts development possible around/after 21z.

/Outlook for kmsp/

Friday...VFR with MVFR/thunderstorms and rain late. Winds NE at 10kts.
Sat...MVFR with IFR/tsra+ possible. Winds east at 10g15kts.
Sun...VFR with MVFR/thunderstorms and rain possible. Winds northwest 10kts.


Mpx watches/warnings/advisories...


long term...dwe

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