Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
905 PM CDT sun Jul 5 2015
No big changes to forecast except to add some patchy fog into the
late overnight hours. Latest RUC soundings Show Low-level winds
decoupling somewhat...and even though some turbulent mixing just
above modest near-surface inversion will prevent any widespread
fog...could not rule out some patchy fog. Any fog should dissipate
within an hour or two of sunrise. Looking at trend of latest hrrr
and previous NAM runs of bringing a lead band of dying convection
from storms...currently over Minnesota back to southeast South
Dakota...across southern Wisconsin in the middle to late morning
hours. Next shift will have to assess the full 00z data set to see
if there will be a need to bring some probability of precipitation into the area in the
Except for low chance for some patchy fog in the last few
hours before sunrise...still looking at trend of latest hrrr and
previous NAM runs of bringing a lead band of dying convection
across southern Wisconsin in the middle to late morning hours. Will
await a look at complete 00z data set to see if there is a need to
account for this initial band in the 06z tafs. 12z guidance was
showing potential for sub-1000 feet ceilings along and behind the front.
Will await full data set to see if this trend continues.
Latest short-term guidance is supporting current forecast trend
of southerly winds increasing through the morning...and thus will
leave timing of Small Craft Advisory as is. Prior to winds picking
up...could see some haze/fog that reduces visibilities under 5
miles...and possibly some patches of dense fog.
There is a potential for a high swim risk for the beaches of
Sheboygan and Ozaukee counties from late tomorrow morning into
Monday evening. Strengthening south winds in the tightening
pressure gradient ahead of an approaching cold front will produce
3 to 5 foot waves and life-threatening currents...especially on
south-facing beaches and near piers and breakwalls. Next shift
will have to assess latest guidance to see if a beach hazard
statement will be needed.
Previous discussion... /issued 330 PM CDT sun Jul 5 2015/
Tonight - confidence...medium.
Weak middle level southwest flow with broad low level anticyclonic
regime in place. Warm and increasingly moist southerly flow setting
up ahead of the system in the plains. Low level jet and shortwave activity
will keep the storms well to our west this period. Diurnally driven
cumulus will dissipate this evening with potential for convective debris
arriving as the night wears on.
Monday - confidence...medium
A warm and humid day is setting up as south-southwest winds become gusty. 925
temperatures off the NAM reach levels that would support a few lower 90s in
parts of the east. Other models not quite as aggressive and some
concern about lingering debris clouds from convection to our west.
So won't go quite that high but will have plenty of middle/upper 80s
going. Westerly 500 millibar flow increases with shortwave proggd
into western WI during the afternoon. Instability looks to be plentiful. 12z
NAM soundings support convective available potential energy to near 3000 j/kg in msn during the afternoon
while the 12z GFS has increased from the 6z run...with convective available potential energy around
1000 j/kg. If sun is ample think we can hit at least somewhere
closer to The Middle Ground. So middle level forcing will be arriving
in prime time. Cwasp numbers highest on the NAM...in the low
70s...while other models lower. 0-6km shear is OK but not at the
Sweet spot of at or above 40 kts. However 0-1km shear is 20-25 knots which
is favorable. Some concern over the proggd very low middle level lapse
rates and the orientation of 850 jet core showing more of an
entrance region placement at 00z. Main upper jet is proggd to our
north but some increase in upper divergence noted during the
afternoon. 12z European model (ecmwf) develops a stronger low into central WI by 00z
with other solutions a bit weaker but best low level frontal
convergence just west of County Warning Area but drawing closer during peak heating.
Will have highest probability of precipitation aligned similarly to what we have going with
biggest probability of precipitation in the northwest and smallest chances in the southeast. 4km
Storm Prediction Center WRF does suggest a scenario similar to the 12z NAM with some
development possible during the afternoon ahead of the afternoon.
Storm Prediction Center has upgraded to slight risk for much of County Warning Area and this seems
reasonable especially given the potential instability and better forcing
parameters arriving at peak time of day into especially SC WI. Given proggd
poor middle level lapse rates would not expect much in the hail
department...which cips analogs support...with wind event more
likely given favorable lower level lapse rates and pretty decent
downdraft convective available potential energy proggd.
Monday night through Wednesday...forecast confidence - high
trending toward medium.
Will continue to carry a 3 to 5 hour period of categorical probability of precipitation
across County Warning Area through 06z Tuesday as strong cold front moves across southern WI.
Short term guidance in better agreement on carrying axis of higher
layer q-vector convergence across southern WI associated with upstream
short wave trough currently moving across Wyoming/Colorado area. Plenty of
moisture in place as precipitable water values increase to around 2 inches...which
is about 2 Standard deviations for early July. Moderate
frontogenetical forcing in the low levels as expected with front
will extend up to about 10k feet increasing the liklihood for more
widespread convection. Surface dewpt pooling likely to increase into
the low 70s ahead of the front on Monday. However still some
uncertainty regarding thermal profile aloft and amount of elevated
instability as front moves through. Bulk shear will be
modeate...supporting multicells and perhaps a few supercells. With
surface front moving through during better time of the day...Storm Prediction Center upgrade to
slight risk in western County Warning Area understandable.
Storms end later Monday night with passage of cold front.
Drier...less humid air spreads rapidly across southern WI later Monday night
and continues on Tuesday as high pressure builds into the upper Midwest.
Weak low pressure moving along the front situated south of WI on
Wednesday may brush the area with some middle-high clouds...but at this
time...expecting the drier air and high pressure to keep precipitation
south of the Stateline.
Wednesday night through Sunday...forecast confidence - medium
Medium range guidance having issues late in the week as higher
amplitude pattern transitions to more progressive...zonal scenario.
While deep...closed low over northern Hudson Bay shifts slightly
northward...second weaker closed low over southwest Continental U.S. Is
expected to further weaken and get nudged northeast ahead of
approaching eastern Pacific trough. Guidance having issues with timing
and strength of amount of ridging affecting northern plains into the
upper Midwest late in the week...and its affect on frontal boundary
draped across the central US east into the Ohio Valley. GFS still
bullish on stronger...compact short wave affecting southern WI Thursday night
while Gem much slower and takes this wave well north of the area on
Friday. European model (ecmwf) does carry weak middle-level wave across northern Illinois Thursday night.
For now will continue to carry small probability of precipitation for these periods.
For the weekend...differences continue as European model (ecmwf) continues
drier...more zonal pattern across the upper Midwest while GFS
trending to more southwest steering flow. This would result in more
unsettled conditions for the region as frontal boundary shifts
northward. Wpc blended solution favoring drier European model (ecmwf) solution but
does give some weight to GFS and Gem for next weekend. Hence can
not avoid keeping low probability of precipitation going for the later periods. Temperatures will
warm to more seasonal normals by the weekend.
Quiet and VFR for much of this period. Southerly flow gradually
picks up ahead of frontal boundary approaching from the plains. May
see patchy fog develop once again tonight but not expecting anything
too widespread or dense as south winds at the surface and aloft a
bit stronger. Still looks like the main line of storms arrives later
Monday afternoon in SC WI and arriving early evening in the southeast.
Models are hinting at some pre-frontal development late morning/Erly
afternoon but the main event looks to be from middle afternoon in the SC
through early evening into the southeast.
Marine...have issued a Small Craft Advisory for Monday afternoon
and evening due to a tightening pressure gradient ahead of an
approaching low pressure system and associated cold front.
Lm...Small Craft Advisory from noon to 10 PM CDT Monday for
tonight/Monday and aviation/marine...collar
Monday night through Sunday...mbk