Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
929 PM CST Thursday Jan 29 2015
Flurries pushing through southern WI in bands aligned parallel to
unidirectional flow from the surface to 850 mb...indicating these
are low-topped convective rolls. Forecast soundings show steep
low-level lapse rates and weak cape in the relatively shallow
cloud layer...with some overall weak forcing for ascent provided
by cva as short wave trough drops through southern WI...along with weak
convergence along the associated weak surface trough/wind shift
Also appears low-level moisture is somewhat lake-enhanced from
off of Lake Superior...with some of the bands extending well to
the north toward the lake behind the weak surface trough...though
radar returns are weaker with these northern bands. No snow
reports from surface observation in central and NE WI under these weaker
bands until you get to far north central WI...closer to the Lake
Superior moisture source. This weakening trend in line with the
cloud layer becoming shallow and mixing out as the column dries
from above shown on the forecast soundings due to subsidence
behind the short wave. Should see the flurries exit/end over southern
WI around 06z.
Still a question of how quickly clouds will diminish overnight.
Latest NAM has slowed the progress of clearing...so using a
blended solution keep skies mostly cloudy until 08z-09z...then
diminish the cloud cover for partly cloudy everywhere by 12z. Will
adjust temperatures accordingly but most locations will still get close
to forecasted lows with 925 mb temperatures expected to fall to -13c to
-15c by 12z.
Flurries expected to clear taf sites between 05z and 06z...with
VFR cloud cover scattering out from northwest to southeast between 09z and 12z.
VFR conditions for the remainder of the forecast.
Latest forecasts indicate winds will shift to the north-northwest and subside
through the overnight and into the morning to allow waves to drop
below advisory levels by 18z expiration time.
Previous discussion... /issued 327 PM CST Thursday Jan 29 2015/
Tonight...forecast confidence medium.
850 mb cold air advection continues through midnight or so as
thermal trough starts edging eastward thereafter. Expect plenty of
cloud cover to linger. Some upstream breaks in the stratus with a
few diurnally induced holes here and there. Low level relative humidity forecasts erode
this out after 6z and given that plus increasing 500 millibar negative vorticity advection
after 6z... went with a gradual decrease in cloud cover with the
Friday...forecast confidence medium.
Surface high slides across and largely dominates with lighter
wind regime. However weak shortwaves riding to the north combined
with a smidge of warm air advection should result in a middle deck swinging through
per relative humidity forecasts. NAM soundings look too moist in the low levels
keeping widespread low clouds around. Prefer the drier look of the
GFS with respect to low levels. Upstream clearing supports this
for low cloud trends... trending towards at least a scattered condition
in this regard.
Friday night through Sunday night...forecast confidence medium.
A weak area of low pressure will move southeastward across southern Ontario
Canada Friday nt and Sat. Southwesterly surface flow and low to middle level warm
advection...along with some clouds...will keep temperatures from
dropping too much. The warm advection and possibly some periods of
sunshine on Sat will support high temperatures in the lower 30s. The cold
front will pass gradually from the late morning through the
afternoon but the stronger cold advection will not arrive until
late Sat evening when brisk northeasterly winds develop. The northeasterly flow and 850
mb temperatures dropping to -11c to -15c by 12z sun will support the
potential for lake effect snow. At the same time...a shortwave
trough will dig to eastern NE by 12z sun and across central or southern Illinois
sun evening...with the rrqd of a 150 knots westerly jet over southern WI on sun. The
consensus of models puts the WI/Illinois border on the northern fringes of
this system. Thus 50 probability of precipitation are forecast over far southern WI due to the
system with 50 probability of precipitation in the east due to the lake effect snow
potential. A dry northeasterly boundary layer flow will likely limit the
intensity of any lake effect snow bands especially toward east
central WI. The current snowfall forecast for Sat nt and sun is
for an inch near the Illinois border and 2-3 inches in far southeast WI where
the combination of system snow and lake effect snow will occur.
The snowfall forecast will change as the details of the system
snow and lake effect snow become more clear. Cold temperatures will
return to southern WI for Sat nt through sun nt.
Long term...Monday through Thursday...forecast confidence medium.
Polar high pressure to prevail on Monday with continued cold temperatures.
Southwesterly flow and warm advection to return for Monday nt and Tuesday as a
shortwave trough moves across the state. A much stronger upper
trough will then dig into the central USA and progress to the eastern
USA for Wed-Thu. A trough of low pressure will pass through the
area ahead of this feature for Tuesday and Tuesday nt. Snow chances are
forecast from Monday nt-Tuesday nt. Strong cold advection on northwesterly winds
to then prevail late Tuesday nt-Wednesday nt. A strong polar high to follow
for late on Thursday. Highs in the teens for Wednesday-Thursday with lows in the
single digits below zero to single digits above zero.
Aviation/00z tafs/...MVFR deck remains in place into the evening
hours with continued cold advection. Northwest winds gradually ease
tonight...especially after 6z as surface high nudges closer. The
high dominates Friday with VFR conditions and lighter wind regime.
The start of some weak warm air advection and shortwaves riding to our north should
spread middle clouds across southern WI.
Marine...will continue small craft but extend a bit longer to allow
a little more time for wave cessation. Northwest winds are expected
to gradually ease as the night wears on with high pressure nudging
closer. However gradient region remains near lake which will likely
allow higher waves to continue especially a bit closer to the open
waters. Higher waves will likely subside by midday Friday.
Lm...Small Craft Advisory until noon CST Friday for lmz643>646.
tonight/Friday and aviation/marine...collar
Friday night through Thursday...gehring