Scientific Forecaster Discussion
fxus63 kmkx 210454
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1154 PM CDT Thu Jul 20 2017
Aviation(06z tafs)...areas of ground fog are forming and will
continue to do so although thick cirrus from tstorms moving
across Iowa and Illinois may limit it. Sct cumulus around 3-4 kft and
sct-bkn mid level deck will then prevail for the afternoon.
Showers and tstorms will begin to increase in coverage from west
to east through the afternoon and evening with the strongest
storms Fri nt. Cigs and vsbys greatly reduced within tstorms. Low
stratus possibly below 1 kft will develop after the storms for Sat
Previous discussion... (issued 857 PM CDT Thu Jul 20 2017)
Update...only maintaining a slight chance of thunder late tnt in
SW WI with tstorms to likely remain in Iowa and Illinois. Some ground fog
may form depending on extent and opaqueness of cirrus clouds from
convection to the south. Otherwise, weak 850 mb warm, moist
advection spreads from west to east across srn WI from the late
morning through the afternoon. Some showers and tstorms will
occur during this time with best chances over south central WI,
but the main focus for deep convection will be Fri nt. For Fri nt,
a strong swly low level jet will focus toward west central WI in response
to a weak shortwave trough and a right entrance region of a jet
streak moving across the nrn Great Lakes. The convection over
central WI will the likely form into a strong to severe mesoscale convective system and
move sewd through srn WI for the late evening and overnight with
the low level jet veering during this time. There will probably be some
storms ahead of the mesoscale convective system over srn WI as weaker warm advection and
moisture transport will be ongoing. Also cannot rule out some
backbuilding on the wrn flank of the mesoscale convective system as the low level jet veers wly.
The Flash Flood Watch is well placed over saturated soils and
already high rivers.
Aviation(00z tafs)...areas of ground fog may form during the
early morning hours on Fri as high pressure moves across the
region although thick cirrus from tstorms moving across Iowa and Illinois
may limit it or prevent it from forming. Sct cumulus around
3-4 kft and sct-bkn mid level deck will then prevail for the
afternoon. Showers and tstorms will begin to increase in coverage
from west to east through the afternoon and evening with the
strongest storms Fri nt. Cigs and vsbys greatly reduced within
Marine...relatively light winds and low wave heights for Fri-Sat.
A humid airmass may lead to some fog over Lake Michigan.
Previous discussion... (issued 306 PM CDT Thu Jul 20 2017)
Today and tonight...forecast confidence is medium.
The stationary frontal zone will remain draped across far southern
Wisconsin into northern Illinois tonight. The best low-level
frontogenesis and surface convergence will remain to our south,
although a shower or thunderstorm may brush our southern counties
after midnight. Areas of fog are possible tonight given partial
clearing, light winds, and low dewpoint depressions. Temperatures
will remain mild.
Look for the frontal zone to lift north into southern Wisconsin on
Friday, with moisture transport increasing from late afternoon
into evening. Deep layer moisture will rapidly increase from the
southwest as the front lifts north, with precipitable water values
climbing from around one inch Friday morning, to nearly two inches
late Friday afternoon. Available moisture parameters point towards
high precipitation efficiency, with near parallel mid to upper
flow along the frontal zone. Given recent heavy rainfall amounts,
went ahead and issued a Flash Flood Watch for our southern and
western areas from Friday afternoon into Saturday morning. Given
the amount of wind shear and instability, some of the
thunderstorms could become severe late Friday afternoon, producing
damaging winds and large hail.
Friday night and Saturday...forecast confidence is high.
As mentioned above, the frontal boundary will lift northeast into
southern Wisconsin by Friday evening with a decent low level jet
oriented and strengthening across it. The moisture feed is
impressive with precipitable water values forecast to be around
2-2.25 inches. The scenario is similar to the last few heavy rain
events. We'll likely see a large convective complex develop and
slowly roll southeast across the area Friday evening. The best
cape axis will likely stretch from southeast Minnesota southeast into southwest
Wisconsin. Deep layer shear is marginal at around 30kts...llv
shear is better. Propagation vectors Friday evening suggest slow
movement, or back building potential.
Our expected axis of heavy rainfall will run south of a Baraboo to
Lake Geneva line, but it will be heavily influenced by how this
convection evolves. Remain alert to changes as this situation
develops. A Flash Flood Watch will remain in effect through 9 am
Saturday morning for parts of southeast and south central
Wisconsin. Mainly the areas that were hit hard last night and over
the past 7 to 10 days.
The rain should be exiting toward sunrise on Saturday. The NAM
wants to develop more convection Saturday afternoon. However, it's
pretty much an outlier with the amount of rain it develops. We
could see some showers or storms, but support should be
diminishing as the low pressure passes off to the east.
Sunday through Wednesday...forecast confidence is medium.
Thankfully, we should enter a dry period for much of the first
half of next week. Temperatures will be cooler with lower
Thursday...forecast confidence is medium.
The long range models show a system dropping through later
Wednesday night into Thursday, bringing our next chance of rain.
Lake breeze has penetrated a few miles inland, resulting in
easterly winds near Lake Michigan. Otherwise light north to
northwesterly winds will prevail, nearing calm later tonight. We
may see areas of fog tonight given the amount of low-level
moisture present. The biggest weather related impacts will be
shower and thunderstorm chances which will increase from late
tomorrow afternoon into tomorrow night. The stronger storms could
produce large hail and strong gusty winds.
North to northeasterly winds generally under 15 knots will
continue today. Shower/thunderstorm chances will increase from
Friday afternoon into Friday night. Prevailing winds and waves
should remain below Small Craft Advisory levels, although higher
winds and waves are possible in thunderstorms on Friday and Friday
WI...Flash Flood Watch from Friday afternoon through Saturday morning