Scientific Forecaster Discussion
fxus63 klot 231218
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
618 am CST Fri Feb 23 2018
157 am CST
Scattered showers are approaching the I-39 corridor and more
widespread showers will spread east across the forecast area early
this morning. 0.1-0.2 inches of rain are expected from the showers,
but the additional rainfall should have little to no impact on area
rivers. Areas of fog are also being reported at most locations, but
dense fog is not expected.
The weak surface low will shift north over the northern Great Lakes
while the low's front shifts northeast. Rain will shift out of the
forecast area late this morning, but cloudy conditions will
continue. High temperatures will be in the mid 40s to around 50.
High pressure spreads over the upper Midwest tonight as a frontal
boundary and weak upper level disturbance shifts north into central
Illinois. Guidance differs on how far north the precip associated with
this front will get, but I think precip will be along and south of I-
80 by sunrise Saturday morning.
A wintry mix of precip including freezing rain is possible. Forecast
soundings feature a deep warm layer and very little saturation in
the dgz. Therefore, any precip that falls should be liquid prior to
hitting the ground. Surface temperatures will be key, and right now
surface temps are forecast to be right around freezing along the I-
80 corridor and points north. Therefore, there could be slick spots
Saturday morning, especially on colder, elevated surfaces.
Temperatures will warm to above freezing by mid to late Saturday
morning, ending the ice threat.
I'm not expecting much ice accumulation so I will not be issuing any
ice related headlines.
157 am CST
Saturday through Thursday...
Overview...a wintry mix is followed by more rain and thunderstorms
Saturday and Saturday night. Rain from this system could cause rises
on local rivers. Strong winds, possibly gusting to around 40 mph,
are possible Saturday night. Drier and relatively mild conditions
are expected early next week, but rain may return mid to late week.
The aforementioned Saturday wintry mix turns to all rain by mid to
late Saturday morning. Scattered light showers could reach the Illinois/WI
state line, but I have higher confidence in measurable precip along
and south of I-88.
The next surface low takes shape over the High Plains Saturday, and
reaches the upper Mississippi River valley Saturday evening.
Scattered showers are possible Saturday afternoon, although some
models suggest we could have a dry period. As the low itself moves
into the region, rain, potentially hard at times, will spread across
the region. Similar to earlier this week, embedded thunderstorms are
possible Saturday evening. While not as high as earlier this week,
pwats are forecast to be well above normal at around 1 inch.
Rainfall totals from late tonight through Saturday night will be 0.3
inches or less north of I-80 and 0.3-0.6 inches south of I-80 with
locally higher amounts. The additional rain could lead to minor
rises on rivers, but nothing as significant as what we experienced
earlier this week.
Gusty west winds are expected Saturday night into Sunday. The GFS
would suggest gusts of 40-45 miles per hour are possible while the NAM has
gusts of 30-35 miles per hour. Either way, Saturday night looks windy.
The beginning of next week looks drier with a warming trend.
Additional systems could bring rain to the region Tuesday night
through mid next week, and models are hinting that the system next
Thursday could bring another round of moderate to heavy rain.
for the 12z tafs...
Rain will be tapering to drizzle then moving out of the area this
morning but low vsbys and especially low cigs will linger at
least into mid morning. Winds will take on more of a southwesterly
component and begin to gust a bit by afternoon. The next chance of
precipitation looks to arrive by mid morning Saturday, perhaps
with a period of mixed wintery precipitation followed by a
changeover to rain by afternoon. Opted not to include this in the
present set of tafs given the active weather in the short term and
uncertainties about timing and exact trends in precipitation
types tomorrow morning.
336 am CST
The primary concern for the next few days will be the passage of a
rapidly deepening low pressure system Saturday night. Strong
southeast winds Saturday evening are expected to veer to
southwesterly and increase in speed by Sunday morning. There is
some disagreement between forecast models as to how strong the
winds will get, but gales appear to be likely and high end gales a
possibility. There was also consideration of storm force chances,
but presently that appears less likely. While the pressure
gradient may be favorable, the mild southerly winds would not mix
down very effectively over the Cold Lake surface. The same
pressure gradient with cold north winds over a warm like might be
a different story. Winds then relax a bit by Monday morning.
lmz779...6 PM Saturday to 3 am Monday.
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