Scientific Forecaster Discussion
fxus63 kmpx 162230
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
530 PM CDT Mon Sep 16 2019
Update...for 00z aviation discussion below
Short term...(this evening through Tuesday night)
issued at 338 PM CDT Mon Sep 16 2019
A Summer-like day out there this afternoon as temperatures have
warmed into the mid to upper 80s, and dewpoints into the upper 60s
and low 70s, as a warm front works through he region. Strong low-
level warm air advection will continue tonight, with isentropic
ascent along the nose of a developing low-level jet creating
enough forcing for scattered showers and thunderstorms to develop
overnight. Expect precipitation to begin across the eastern
Dakotas and western Minnesota shortly after midnight, with showers
and thunderstorms spreading northeast into central Minnesota and
west- central Wisconsin through the morning and barely afternoon.
Shear and lapse rates look rather paltry with the thunderstorms
tonight, so no severe weather is not expected. Lows will stay
quite warm overnight with widespread cloud cover and continued
southerly winds, with temperatures only falling to the low 70s
across western Minnesota and mid 60s across west-central
Another warm day is expected tomorrow under continued low-level
thermal ridging and strong southerly flow at the surface. Highs
will again warm into the low to mid 80s across central
Minnesota/west-central Wisconsin, and upper 80s across southern
and western Minnesota. Winds will again be gusty out of the south,
as a surface low over the Canadian prairies deepens to our
northwest. The cold front from this low approaches late Tuesday
night, and will be the focus for widespread showers and
thunderstorms as the low-level jet again strengthens overnight and
overruns the front. A line of thunderstorms is expected to
initiate rather quickly along the boundary shortly after midnight,
and progress eastwards through the morning. Given the July-like
airmass overhead, it's no surprise that precipitable water values will be near the
climatological Max for mid-September, with values above 1.75"
likely. Thunderstorms will likely be efficient rainfall producers
with wet-bulb zero levels above 10-12 thousand feet along with the
high precipitable water values. The progressive nature of the front should limit
the overall amounts, but still expect widespread amounts of 0.5-1"
by Wednesday morning, with localized amounts closer to 2".
Additionally, much of the area is under a marginal risk of
thunderstorms Tuesday night-Wednesday morning, and MUCAPE of
1000-1500 j/kg along with effective bulk shear around 30 kts would
be sufficient for organized thunderstorms. Some limiting factors
will be the better deep shear lagging behind the cold front and
only modest lapse rates, but some isolated instances of large hail
and damaging winds look possible with the strongest updrafts.
Long term...(wednesday through monday)
issued at 338 PM CDT Mon Sep 16 2019
A southwest flow pattern will persist through Saturday, but
flattening of the southeastern U.S. Ridge Sunday and Monday will
allow cooler air to return. Until then, very warm and humid
conditions will continue, along with periodic thunderstorm
chances. There have been only 3 days since 1945 between sept
10-20 where dew points reached 72 or higher at msp. We could add
another couple days to that total this week.
The cold front will sag southeast to WI and Iowa by Wednesday
morning as the brunt of the energy lifts northeast into Canada.
Cold air advection will be weak or non-existent so temperatures will remain
seasonably warm in the low 80s Wednesday and Thursday. A few
storms could develop near the front as well.
The stalled front will lift back north Thursday night with an
impressive trough lifting northeast into the northern plains.
Moisture transport will be confined to the plains during the
evening, but overnight it will work eastward into Minnesota. Scattered
thunderstorms are possible, but the best chance will come Friday
and Friday night as the cold front and trough approach. Strong
forcing with the front and fairly unidirectional flow along the
front will favor squall line development. While bulk shear is a
little less than impressive with the strongest 500 mb flow well
west of the cold front, the introduction of a 40-45 kt low level jet
Friday evening will bring an elevated risk of wind damage.
Highs Friday are a bit in question due to the potential for
clouds and thunderstorms, but a well-mixed boundary layer and deep
tropical flow streaming northward favor very warm and humid
conditions. Raised highs several degrees above the nbm to the mid
80s Friday, and raised them a few degrees for Wednesday and
Thursday as well due to a recent cool bias with the nbm.
Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 530 PM CDT Mon Sep 16 2019
Will have a broken-overcast deck of mid/high clouds through the
period. An area of showers and thunderstorms is forecast to work
into west central/central Minnesota overnight. Have maintained a mention
at kaxn/krwf/kstc with MVFR conditions, but expect other sites to
remain dry. Southeast winds through the period, with surface
gusts diminishing this evening (except at western sites-kaxn and
krwf). Will have low level wind shear overnight at Minnesota sites where
surface gusts diminish. Surface gusts redevelop at all sites
around 15z Tuesday.
Kmsp...expect measurable precipitation to stay north/west of site.
Higher chance for rain showers/ts arrives after 06z Wed.
/Outlook for kmsp/
Wed...VFR. Chc am MVFR/tsra. Wind SW 10g20 kts.
Thu...VFR. Chc MVFR/thunderstorms and rain late. Wind southeast 5 kts.
Fri...VFR. Chc MVFR/tsra. Wind southeast 10g20 kts.