Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
356 PM CDT Thursday may 28 2015
Tonight and Friday... forecast confidence medium.
A shortwave over Iowa and northern Missouri is producing showers and
thunderstorms this afternoon. This shortwave will swing
northeastward into southeast Minnesota and SW WI overnight. It will be
accompanied by a fairly weak low level jet. This feature will bring
low level Theta-E advection across southern WI late tonight. There
is a strong signal for moisture return but weak warm air advection.
Expect rain to hold off in the mkx forecast area until after
midnight. Then it should spread through the area from late tonight
through middle Friday morning. Expect a lull in precipitation activity midday
between shortwave and advection activity.
The low level jet will begin point into into southern WI late Friday
afternoon. Models are showing afternoon development in southeast
WI... perhaps along a weak surface convergence zone. Cape will be
decent...up to 1000 j/kg... during Friday afternoon with moderate
bulk shear around 30 knots. There is a potential for a couple of
stronger storms to develop Friday afternoon. Storm Prediction Center highlighted our
area in a marginal risk for severe weather.
Friday night through Sunday...forecast confidence - medium.
Enhanced synoptic scale forcing slides through southern Wisconsin
Friday night. Frontogenetical forcing along cold front and
strengthening right entrance region of upper jet will affect the
area during this time. By 00z/Sat...precipitable water values will have climbed
to around 1.5 to 1.7 inches which is estimated to be reaching 90th
to 99th percentile for middle to late may. Storm motion expected to be
from west-southwest around 25 kts however with axis of low level jet with
southwest 25kt winds...resultant mesoscale-Beta elements demonstrate slow
movement of 5 to 10 kts. Fortunately...3 hour ffg most areas around
For now...will mention heavy rainfall potential in severe weather potential statement. Cwasp values
over 70 percent Friday evening as cold front approaches from the
northwest. 0-6km bulk shear approaching 30 kts with SBCAPE
increasing to over 1000-1500 j/kg. Minimal criteria for severe so
marginal risk looks appropriate.
Leaning more on 12z GFS/European model (ecmwf) solutions as NAM showing stronger middle-
level circulation over southwest WI/tri-state area into Saturday.
Other guidance shows flatter...more progressive flow with middle-level
open trough exiting southeast WI later Saturday. Hence will continue
shower threat into Saturday morning before drier air overwhelms low
levels and synoptic scale forcing slides off to the southeast.
925h temperatures fall to around 3-4c later Saturday night. With clear
skies...expect minimum temperatures to fall into the upper 30s to lower
40s. Will hold off on frost mention at this time due to boundary
layer mixing. Signficantly cooler for the weekend with 925h temperatures
collapsing 10-15c from Friday.
Sunday night through Thursday...forecast confidence - medium.
Medium range guidance in general agreement on upper level zonal flow
at the start of the period transitioning to higher amplitude ridging
over central Continental U.S. Into Tuesday as long wave trough spreads into
western Continental U.S.. hence quieter...drier conditions associated with surface
ridge that will have settled in later in the weekend will persist
through the first part of next week while low level jet and deeper
moisture return focuses across the northern plains...perhaps
affecting northwest WI.
Expect a gradual warmup during the period as the low level flow
turns more southerly...with the Cool Lake Michigan waters continuing to
cool the Lakeshore counties. Ridging over most of the Great Lakes
and eastern Continental U.S. Begins to break down by midweek as stronger low
pressure moves through the plains. Hence increasing chance for
convection beginning Wednesday night and continuing through the end of the
week. At this point...best chance for more widespread convection
around Friday/Sat as southward extension of low pressure passes across
western gtlakes. GFS 500h 5-day means continue to show near or
slightly above normal heights over the gtlakes through the first
week of June.
VFR conditions today and tonight. Middle clouds increase late tonight
as moist and unstable air returns to the region on a southwesterly
low level jet stream. There should be a round of showers and
thunderstorms that spread through southern WI from late tonight
through Friday morning... a lull midday... and then more development
later Friday afternoon.
Heavy rain is the main threat for late Friday afternoon through
Saturday morning in response to a cold front tracking through the
region. There is a small threat for severe wind gusts and hail.
A cold front will drop Down Lake Michigan on Saturday and bring
gusty northeasterly winds. Winds and waves are expected to be at
Small Craft Advisory levels.
Tonight/Friday and aviation/marine...mrc
Friday night through Thursday...mbk