Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
553 am CST Friday Mar 7 2014
328 am...forecast concerns include high temperatures just about
every day over the next week...potential for minor snow
accumulations late tonight into Saturday morning and another
period of snow accumulation Tuesday night into Wednesday.
High/middle level clouds will be increasing later this morning and
will likely become broken/overcast by middle/late afternoon. While this may
have an impact on high temperatures today...strong warm air advection
will take place with winds turning southwest. Current temperatures across
northern MO/southwestern Iowa are in the Lower/Middle 30s. Thus have
continued with current forecast of highs generally in the Lower/
Middle 40s...perhaps a few upper 30s across northern areas and then
possible upper 40s/near 50 far south where snowpack has melted.
A cold front will enter the northwestern County Warning Area this evening...
moving south of the area by Saturday morning. After a quick drop
with sunset...temperatures will likely hold in the middle/upper 30s until
frontal passage. Once winds turn off the lake...colder air over the ice pack
could usher in slightly colder air than might otherwise be
expected. Thermal profiles support precipitation type quickly switching
from rain to snow. There could be a brief period where precipitation type
mixes with some sleet or surface temperatures drop below freezing before
precipitation switches over to snow. Have maintained just rain to snow in
the forecast but late evening through early overnight will need to
be monitored for a possible mix.
Gem/ECMWF/GFS all in good agreement with a band of light quantitative precipitation forecast
moving across the northern half/two thirds of the County Warning Area overnight
into Saturday morning with the bulk of this falling as snow. Thus
snow accumulations of up to or around an inch look on track though
confidence on specific amounts is a bit lower due to surface temperatures
and snow/water ratios. Snow could be rather wet as it begins with
some initial melting.
This precipitation shifts southeast by Saturday afternoon with dry
weather expected through Monday night along with a warming trend.
A ridge of high pressure will settle across the region Saturday
night into Sunday morning. Light winds and clear skies should
allow for good radiational cooling. Have held temperatures close to
previous forecast...warming them a few degrees. But if the high
speeds up and flow turns southwesterly faster...warmer air aloft
may keep low temperatures warmer than currently advertised and this
trend will likely continue into Sunday. With sunny skies expected
have increased temperatures a few degrees toward warmer mav numbers and
this may end up still being too cool...currently middle/upper 30s
north to middle 40s south.
Gem/European model (ecmwf) are now in decent agreement with 850mb temperatures into the
+10c to +12c range for Monday and made no changes to going highs
of middle 40s north to middle 50s south as these appear on track at this
A cold front will move south Monday night into Tuesday and its a
bit early for specific timing but temperatures ahead of this front will
stay mild and there could be quite a temperature gradient over the County Warning Area or
nearby on Tuesday. Possible that temperatures across the southern County Warning Area
will be back in the 50s ahead of the front on Tuesday. During this
same time period...a storm system will be developing over the
plains and shifting east across the Ohio Valley in the Wednesday
time period. Current trends would suggest the heaviest precipitation
from this system will remain south of the area...with a period of
rain changing to snow with the cold front...similar to tonights
system. However...made no significant changes to Tuesday/Wednesday.
//Ord and mdw concerns...updated 12z...
* SW winds over 10 knots this afternoon.
* Chance for a light mix of precipitation late this evening into early
overnight before light/moderate snow towards daybreak Sat.
* Winds becoming northeast overnight with MVFR ceilings arriving.
High pressure will continue to move east away from the area
allowing for south southwest winds to kick in and for early
morning MVFR fog to dissipate. A few gusts around 15 knots are
possible. A cold front will ease its way across northern Illinois and
northwest in this evening. This front continues to show more anafront
characteristics...that is the forcing for precipitation is located behind
the actual surface wind shift. This forcing is primarily light
however there does appear to be an upper disturbance to pass late
tonight into early Saturday morning that would focus lift and
convergence for more maximized coverage. If precipitation can develop
early enough it could be rain or ice pellets but would likely be
light prior to 07z-09z. Substantial enough cooling should occur
for precipitation type to be snow when the stronger forcing translates
over the area late tonight into early Sat morning. Forcing will
shift east southeast during the morning Saturday. Snowfall amounts
are presently favored to be up to an inch at most.
With the cold front tonight will come a wind shift to northeast
and with that could arrive MVFR clouds soon thereafter. By early
overnight...the wind direction will be more favored to turn north
northeast or even northeast at chicagoland airports due to the
Lake Michigan influence. With this...speeds are also favored to
come up over 10 knots late tonight into the morning Sat.
//Ord and mdw confidence...updated 12z...
* medium in snow timing and low in visibility forecast during
* Low in timing of wind shift to northeast tonight.
//Outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night-Monday...VFR likely.
Tuesday...chance of rain during the day and snow during the night.
220 am CST
The pattern over Lake Michigan will transition back to more active
as several systems will pass over or near the Great Lakes in the
next week. By tonight...low pressure from the upper Mississippi
Valley will advance to southern Ontario and bring a cold front
southward. While the low is not moving that quickly...the front
should progress nicely down the lake tonight as anafronts often
can this time of year. High pressure will build into the plains
states Saturday but its center will evolve more south than east
as an active Pacific jet advances eastward along the U.S./Canadian
border. This will steer two low pressure systems quickly eastward
with each favored to pass just north of Lake Superior...one Sunday
and the other Monday afternoon into night. With each will come
warm air advection. With so much of the lake being ice
covered...and snow on top of that ice...a Stout inversion is
favored. So to what degree gusts can reach the surface is
challenging. At this point the first system would be more favored
to bring strong winds and at this time have kept gusts below gales
in the forecast...but the north and central parts could end up
being close to at least temporary gales.
The visible satellite from Thursday revealed the lake is mainly
ice covered with the National Ice Center indicating that is the
case over 90 percent of Lake Michigan. So wave forecasts are only
for a small part of the open water and continue to keep out waves
in the Illinois and Indiana nearshore forecast.
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