Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
317 am CDT Wednesday Sep 2 2015
Convection upstream across Minnesota south into Iowa associated with a long
advertised middle level short wave forecast to push east very slowly
over the next 24 hours. It will likely just be exiting eastern
Wisconsin around 12z Thursday morning. A moderate low level jet is
currently bringing decent Theta-E/moisture advection in the 850-
700mb layer. The models handle the progression of the entire system
quite differently. Interestingly...most of the hi res models...
nmm/arw/hrrr...keep the bulk of the convection north of the County Warning Area
through the afternoon and into the evening. The NAM/GFS/ECMWF...are
much more bullish in allowing the convection to eventually turn
southeast and push across mainly the northeast half of the forecast
area. This occurs already by this afternoon. As Storm Prediction Center mentions in
their day one outlook...which has a marginal risk across our
north...the overall pattern is...and I paraphrase with a non-
word...meh. The surface features a weak...nondescript pattern and
the low level jet weakens with time. The middle level short wave
remains the main driver along with ample moisture in place. This is
a situation where the convection will drive the pattern...not the
other way around. So...would expect the convective complex to
continue to push into northern/central Wisconsin...then turn
southeast with time due to cold pool influences. This puts our northern
forecast area under the highest probability of precipitation later this afternoon and
tonight. If the convection is robust enough...we could see some
strong/severe wind gusts. The areas southwest of Madison could miss
out entirely on this event.
Temperatures are tricky today and mainly due to the threat of cloud cover
from the convection. 925mb temperatures are still in the middle 20s celsius
range and with full sunshine...we/d have hot temperatures like yesterday.
But...will back off slightly given the high/middle level clouds likely
to start spilling in here later this morning and afternoon.
Thursday through Saturday... forecast confidence medium.
The shortwave drifting across the top of the extensive ridge over
the center of the country is expected to stall over Lake Michigan on
Thursday and might even retrograde back into WI Thursday night. Any
convection from Wednesday night may lay out a cold pool. Models are
hinting at a situation where new convection develops along the cold
pool boundary somewhere in central/eastern WI. The GFS is most
bullish with Thursday night while other models are drier. Given a moist
atmosphere with precipitable water values up to 1.7 and decent
elevated cape... any sort of weak forcing such as the cold pool
boundary or retrograding shortwave could produce a few
showers/thunderstorms. Thus... kept the slight chance probability of precipitation going
through much of this period.
925mb temperatures will continue to be in the 23-25c range... warmest
toward western WI. Clouds will be a factor with the maximum temperatures... but
went with highs around 80 near the lake and upper 80s well inland.
Sunday through Wednesday... forecast confidence medium.
A strong upper low will eject out of the Pacific northwest and
temporarily break down the ridge over the upper Midwest early next
week. This will bring shower and thunderstorm chances back to
southern WI mainly Sunday night into Monday with a weak cold front.
Models diverge late in the period so low confidence beyond Tuesday.
The GFS stalls the front near the WI/Illinois border and then drops it
south on Wednesday. The European model (ecmwf) moves the front down through Illinois
already on Tuesday but then brings another wet system through the
upper Midwest on Wednesday.
Some early morning fog could bring reduced visibilities...but this
will likely be gone by 13z this morning. Then look for mainly VFR
conditions through the taf period. There is a chance of showers and
thunderstorms mainly tonight. Brief MVFR conditions are always
possible within this activity. The strongest activity should stay
north of the taf sites.
Today/tonight and aviation/marine...Davis
Thursday through Tuesday...mrc