Scientific Forecaster Discussion
fxus63 kmpx 200903
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
403 am CDT Tue Aug 20 2019
Short term...(today through wednesday)
issued at 402 am CDT Tue Aug 20 2019
The main forecast challenge was trying to determine how the
convection across eastern North Dakota will unfold this morning and
this afternoon. Will is move more easterly and weaken during the
afternoon, or will it dive more south and March along and north of I-
94 while maintaining a severe threat during the morning? Both
scenarios are plausible, but at this time slightly favor the second
solution, with a severe threat along I-94 this morning.
The reason being, there is a Stout elevated mixed layer aloft, and
later this morning models are in good agreement that dewpoints near
70 will pool in advance of a cold front. Combine that low level
moisture together with late August insolation, and by late morning
MLCAPE values will be around 2500 j/kg or higher in the warm sector.
As for the trigger mechanism, a shortwave trough embedded in
northwest flow will provide the lifting mechanism, and this
aforementioned northwest flow will lead to a favorable deep shear
profile when combined with the southwesterly boundary layer winds.
This near storm environment will favor organized convection north of
I-94 , and the main threats will be damaging wind and large hail.
Hires models are Adamant that thunderstorms will weaken as they
approach western Wisconsin around mid-day, but that seem strange
given how unstable the atmosphere will be.
Looking ahead, much drier air will settle in on Wednesday, with
dewpoints in the 40s to lower 50s, sunny skies, light northerly
Long term...(wednesday night through monday)
issued at 402 am CDT Tue Aug 20 2019
No significant changes to the long term period, which will start out
under beautiful sunny skies and with high pressure arriving from
Canada. Thunderstorms chances remain on track for this weekend, with
the most likely chance of widespread storms coming overnight
Saturday night into Sunday morning.
By Wednesday night, the aforementioned high pressure area will be
sinking in across the upper Midwest bringing dew points in the 40s
and low 50s. Overnight lows by Thursday morning will likely be in
the 40s in portions of central Minnesota and northwest WI, with low 50s
elsewhere. High pressure will maintain control locally into Friday
as it slowly drifts east of our area. Highs in the low to mid 70s
are expected Thursday, warming up 2-3 degrees Friday as a light
southerly wind develops.
By Friday afternoon, a trough moving west to east across southern
Canada will reach western Minnesota spreading cloud cover in from the west
and a slight chance of thunderstorms. The best forcing quickly lifts
north as the trough encounters inherent very dry air from the high
to the east. Guidance continues to suggest this trough will have
trouble producing measurable precip east of the western half of Minnesota.
Nonetheless, it will moisten the atmosphere some ahead of the next
wave quickly on its heels for Saturday night. This second wave
moving in from the west will include stronger forcing from top to
bottom along the frontal boundary and could bring more widespread
thunderstorms to the area. Yet another round of storms will be
possible Sunday night as the main front finally pushes through the
area behind it on Monday. There is a chance of severe weather as the
airmass in place should be sufficiently unstable for strong
updrafts and low level jet enhancement at night both Saturday night
and Sunday night will aid in large scale forcing.
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1112 PM CDT Mon Aug 19 2019
VFR with mostly clear skies this evening. Rain/thunderstorms and rain chances develop
overnight at axn/rwf/rnh and after daybreak at stc/msp. Ceilings
will begin to lower overnight with MVFR conditions expected by
tomorrow morning at Minnesota terminals and by late morning at rnh/eau.
IFR ceilings appear possible across western Minnesota where ceilings will
be lower. A thunderstorm complex is expected to develop over
southwest Minnesota and generally progress east along the I-90 corridor
overnight, but confidence is low on how widespread thunderstorm
coverage becomes north of this complex through tomorrow morning.
Best chances for rain/thunderstorms and rain will be at axn/rwf/mkt, but have
maintained prob30 mentions at all Minnesota terminals.
Kmsp...ceilings are expected to lower overnight, with MVFR
ceilings expected by mid-morning. Two periods of rain/thunderstorms and rain appear
possible: late morning/early afternoon and then again tomorrow
evening, but confidence remains low in how widespread coverage
will be at the terminal.
/Outlook for kmsp/
Wed...VFR. Wind northwest 5-10 kts.
Thu...VFR. Wind NE 5 kts.
Fri...VFR. Wind southeast 5-10 kts.