Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
553 am CDT Wednesday Jul 23 2014
Short term...(today and tonight)
issued at 338 am CDT Wednesday Jul 23 2014
Most impressive feature on the 500 mb chart last night was the ridge
centered over Colorado...where the 500 mb height measured at Grand
Junction reached the lofty height of 6000 M. Its associated ridge
axis extends north into Saskatchewan...with convergent northwest flow noted
from Manitoba into northwest Minnesota...hence the 1028 mb surface high that is
centered over the Manitoba lakes this morning. During the short
term...the upper ridge will shift east into the upper miss valley
while the surface high will shift over to Lake Michigan by 12z Thursday.
For today...NAM cumulus rule shows scattered-broken cumulus field developing across western
WI...trailing off to little if any cumulus for western Minnesota. However...western Minnesota
will have to deal with remnant cloud cover from upper waves rotating
around the Colorado ridge. In particular...the rap shows the wave
currently over central nodak working toward western Minnesota by the afternoon.
Dry low levels courtesy the 1028 mb high will make it tough to get
the precipitation in the Minot area into Minnesota...though a few sprinkles can
not be completely ruled out. Did boost sky cover west to account for
this wave and its associated cloud cover though. 850 mb temperatures will be
fairly uniform around 12/13c...which has been pretty good this
Summer at producing highs in the 77 to 81 degree range...so no big
changes were needed to the going forecast.
Tonight...as the ridge works across WI...return flow will be
strengthening across the Dakotas...with isentropic lift and Theta-E
advection both blossoming. Low level jet orientation favors activity
developing along the MO river tonight...making about as far east as
southeast sodak...so kept the forecast dry through 12z Thursday...though we
probably will not have to wait very long on Thursday to start seeing
precipitation push into the upper Minnesota valley. Over WI...with the high
overhead the question becomes how low to go...along with the
potential for fog. Did nudge lows down a degree or two closer to the
consensus raw model blend /best performing guidance over the last
week/...which has Ladysmith dipping into the upper 40s. The fog
potential is a bit tougher to pin down at this point...but certainly
if we do see lows down around 50...some patchy fog...especially near
bodies of water will certainly be possible.
Long term...(thursday through tuesday)
issued at 338 am CDT Wednesday Jul 23 2014
Scattered showers and thunderstorms are expected to be over
eastern Dakotas at daybreak on Thursday along and ahead of a
strengthening baroclinic zone stretching northwest-southeast across the northern
plains. The showers and thunderstorms will spread across the Minnesota
County Warning Area during the day and into western WI by evening as the
frontogenetic zone moves east. The forecast area is in the right entrance region
of an 80 plus knot jet segment across the western Great Lakes
with upper level divergence indicated. By the end of the day...a
coupled jet structure begins to develop...along with increasing
divergence...as the left exit region of a new segment arrives from
the eastern Dakotas. This feature then passes across southern
areas of Minnesota and Iowa Thursday night. While this is occurring...low
level moisture transport in the Central Plains veers into Iowa and
southern Minnesota. This should result in thunderstorms increasing in
coverage across the aforementioned areas along with the potential
for heavy rain. The European model (ecmwf) has a great handle on this situation
while other deterministic models struggle to develop much
precipitation...which is a characteristic of warm air advection
events. Hence...chance probability of precipitation during the day Thursday increase to
likely Thursday night across southern areas of Minnesota.
The precipitation will move east-southeast Friday morning with a drying trend
Friday afternoon as a fairly strong short wave moves through.
However...a weak surface front will slowly traverse the forecast area Friday
afternoon and night with some small probability of precipitation remaining. Much of
Saturday looks dry before a compact vertically stacked low
pressure system swings southeast across the upper Mississippi
Valley region Saturday night and Sunday. This necessitates more
chance probability of precipitation for the latter half of the weekend. Highs from
Thursday to Saturday will be slowly rising from around 80 to the
lower and middle 80s respectively. Highs will then trend down into
the middle to upper 70s for Sunday and at least the first half of
next week. The reason for the extended cool down is a deepening
eastern North America trough with a building ridge in the west.
Longer range model solutions have been very consistent on this
development with a 500 mb closed low just north of the Great Lakes
next Wednesday. 500 mb heights across the Great Lakes and Ohio Valley
are forecast to be 2 to 3 Standard deviations below normal.
Accompanying the cooler temperatures will be mainly dry conditions.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 548 am CDT Wednesday Jul 23 2014
High pressure will move across Minnesota and into WI this
period...resulting in quiet conditions with light winds. VFR
conditions will dominate...with the only exception being the
potential for fog this morning and especially Thursday morning over
in eau...when the center of the high will be near central WI. This
will mainly be a River Valley fog though...hence the prevailing MVFR
with a temporary LIFR visible Thursday morning. Other than that...cumulus field
will develop over eastern Minnesota/western WI late this morning...starting out
near 4k feet...but climbing to near 6k feet by the late afternoon.
Out west...middle level clouds out in the Dakotas will spill into western
Minnesota by the afternoon...but expecting nothing more than that from the
Kmsp...high confidence taf with VFR conditions prevailing with light
north winds today...calm winds tonight...followed by light southeast winds
/Outlook for kmsp/
Thursday...VFR. Chance MVFR conds west/ thunderstorms and rain late afternoon and
overnight. Winds southeast at 10kts.
Friday...VFR. Chance thunderstorms and rain. Winds south-southeast at 5-10kts.
Sat...VFR. Winds west at 10kts.
/outlook for kmsp/