Scientific Forecaster Discussion
fxus63 kmpx 180427
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1127 PM CDT Thu Aug 17 2017
Short term...(this evening through friday)
issued at 322 PM CDT Thu Aug 17 2017
Strong cyclone that brought US the heavy rains yesterday is now
filling as it heads across northern WI. We are still in it's moist
cyclonic flow, hence the expansive stratocu field across the area
this afternoon. However, it's becoming quite cellular and we will
see it rapidly shrink across Minnesota the rest of this afternoon and
dissipate in western WI before midnight. Beside the skies clearing
this evening, winds will also back down pretty quick as ridging
for the Dakotas moves in. This will create the main question for
tonight, the potential for fog. We are certainly plenty moist
enough and the cool highs of today with dewpoints still up in the
low 60s means we will have no problem cooling down to the cross-
over temperature. However, the next upper wave is already incoming
over eastern Montana and will be reaching western Minnesota by Friday
morning. This will be spreading an expansive mid/upper cloud field
across Minnesota tonight, which will really limit fog potential. It's
eastern Minnesota into western WI that looks to keep out of the clouds
long enough to have a fog threat, though here, winds just above
the boundary layer look to remain up around 20 kts and may be too
strong to allow for the formation of fog, so have just a
patchy/area fog mention in east central Minnesota into western WI for
As this Montana wave moves across the Dakotas, models are all
similar in showing about a 30 kt low level jet coming up out of Kansas/neb into
southwest Minnesota. Most of the short term models show there will be a
batch of showers going across southern Minnesota through the morning with
the isentropic lift on the nose of the low level jet. Only have chance pops
though as forecast soundings show plenty of moisture above 10k ft,
but are pretty dry below that. Once this morning activity moves by,
attention turns to western Minnesota for the afternoon. With the upper
trough overhead, we will see steeper mid-level lapse rates (7-8
c/km), which will help promote the development of 1500-2000 j/kg of
MUCAPE. Surface winds are weak, with no strong boundary for storms
to latch on to, but given the instability and the deep shear
supplied by the 80-90kt northerly jet streak coming across the
eastern Dakotas, we like the expanded marginal risk across western
Minnesota the Storm Prediction Center has in their day 2 outlook update. Have only chance pops
again for tomorrow where thunderstorm potential looks greatest based
on where cams are generating the greatest amount of convection,
which is west of a Long Prairie to Owatonna line. This also lines up
with where the European model (ecmwf)/GFS/NAM have their highest degree of
Long term...(friday night through thursday)
issued at 322 PM CDT Thu Aug 17 2017
The weather pattern remains active during the long term forecast
period, with shower and thunderstorm chances every couple of days.
The first will arrive late Friday, and the next in the Sunday eve
through Monday eve time frame.
The Friday chances are in response to a shortwave trough speeding
across Minnesota and Wisconsin from the northern plains. Only a weak
trough is progged at the surface/low levels, thus low level
convergence appears rather weak. That being said, steepening lapse
rates and 40+ knots of mid level flow coupled with a pocket of
healthy layer differential divergence nosing into far western and
south central Minnesota could support an isolated severe threat (large
hail and strong winds).
Given the progressive nature of the shortwave trough, skies clear by
Saturday morning, with patchy fog development possible by
daybreak. Temperatures then moderate through the weekend as mid
level ridging prevails with southerly surface winds. Highs
Saturday look to range through the lower 80s, with mid 80s
becoming more common for Sunday highs.
By Sunday evening, model consensus places a frontal boundary on the
doorstep of western Minnesota. This front is forecast to sag across
the area Sunday night and Monday, with a prolonged chance for
showers and thunderstorms. Precipitable water values are forecast to
near 2 inches, so in addition to severe weather, heavy rainfall will
also be a hazard to monitor.
Tuesday through the end of the upcoming work week we see cooling
temperatures as a deep trough over northern Ontario sinks to the
Great Lakes. Highs in the 70s will be common for Wednesday
and Thursday under northwest flow aloft.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1123 PM CDT Thu Aug 17 2017
Departing cloud cover to the east still causing MVFR cigs for Keau
into the overnight hours. To the west, clear skies is allowing for
patchy fog to develop, included a couple hours of fog for kstc.
The next shortwave is already about to push cloud cover and
precip activity into far western Minnesota, main concerns are for rwf
and mkt to see rain showers activity by morning. Rnh and eau could also
see fog develop once skies clear there overnight. Tomorrow will be
much less windy, but thunderstorms and rain will be possible, especially from rwf
through the mkt area where lapse rates are steepest.
Kmsp...thinking precip stays south of msp, continued to leave it
out of the taf. Will have to monitor afternoon potential for thunderstorms and rain
but at this time confidence too low to include in the taf.
/Outlook for kmsp/
Sat...VFR. Wind SW 5 kts.
Sun...VFR. Chance thunderstorms and rain overnight. Wind SW 5 kts.
Mon...chc MVFR/-tsra. Wind S 5 kts.