Scientific Forecaster Discussion
fxus63 klot 261122
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
622 am CDT Sat may 26 2018
320 am CDT
A pattern more typical for mid-Summer will continue through the
Holiday weekend. Early season heat will be an element to note,
and possibly even to daily record levels in some parts of the County Warning Area.
Lake breeze cooling will be slight today and inch up its afternoon
inland penetration Sunday and Monday. While a weakly forced stray
thunderstorm is a possibility this afternoon, Sunday and Memorial
Day look to be dry with high confidence.
Festering convection near the Quad Cities early this morning is
associated with a subtle short wave disturbance, likely with some
convective enhancement by this point, and will gradually fade as
it moves south-southeast. A broader weakness in the mid-level
(500-700 mb) pattern is present this morning across the western
Great Lakes through mid Mississippi Valley, and this trough will
move slowly east-southeast with building heights by afternoon.
Within such a pattern, the probability of convection today is
low, and if it occurs it should be isolated. The aforementioned
wave, along with possibly some outflow from the early morning
convection to our west, could support isolated convection in the
southern County Warning Area this afternoon. Also cannot totally rule out an
afternoon storm popping on the lake breeze with 1500 j/kg of
MLCAPE, supported by dew points in the mid 60s. Otherwise, for the
northern half of the cwa, the convective chance looks really low.
The impressive early season 850mb thermal ridge of around 20c was
analyzed last evening from the Central Plains through Minnesota,
and this will build eastward starting today. Temperatures at that
height are expected to be 17c-19c across the County Warning Area today, and this
correlates with 90 being realized across a majority of the area.
With generally mid 60s dew points during peak heating, heat index
readings will only be about a degree higher than the temperature.
Column winds are less than yesterday, so the lake breeze will make
slightly more inroads during the mid-afternoon, keeping downtown
Chicago slightly cooler with its maximum.
Tonight's lows will only dip into the mid to upper 60s in most
320 am CDT
Sunday through Friday...
The 500mb high is predicted to become closed across Iowa/Missouri
on Sunday, almost in a Rex block fashion north of present
subtropical storm Alberto. This should suppress any convection in
tandem with light low-level flow, and may even limit fair weather
cumulus. The 850mb temperatures will inch up from today too.
Looking at the past 30 years of climatology for 92+ during late
may and early June, these days have correlated with average 850mb
temperatures of 17c, and we are likely to be a degree or two
above that (similar with 925mb temperature comparisons). So
believe that 92-95 is a strong possibility away from the lake.
With such light low-level flow, the lake breeze should begin
earlier and make a little further progress. Memorial Day should be
similar to Sunday.
Dew points are likely not to be a huge factor, thanks in part to
the early season nature and not having evapotranspiration as a
large contributor, and also not a Deep Reservoir of high dew
points to the immediate south being tapped into. So right now have
gone with blended guidance, which mixes afternoon dew points into
the lower 60s, providing heat index readings similar to
temperatures. While lower to mid 90s heat index readings are not
overly eye opening for much of the Summer here, and certainly not
heat headline worthy, the back-to-back-to-back days of this
during both the early season and the Holiday weekend will have US
reminding of proper heat awareness in messaging/graphics.
Beyond, the National Hurricane Center forecast of Alberto takes it
into the mid Ohio Valley by Wednesday evening, though the 00z
global guidance is west of this path (especially the ec). These
models do bring associated moisture with this northward over the
area, mainly late Tuesday night into Wednesday. These also produce
precipitation over the County Warning Area. Confidence on this is quite low given
the system type and diverse spread in GFS ensemble solutions.
Feel that guidance for Tuesday may be influenced by clouds from
this and/or the breaking down of the ridge, and may be too cool
(i.E. Tuesday could end up yielding lower 90s too).
The pattern in the long range beyond Alberto's possible influence
is still one with above normal temperatures favored, and generally
dry at this juncture.
320 am CDT
The following are daily record highs for Chicago and Rockford over
the next few days. At this time, Rockford has the possibility of
reaching today and monday's.
Saturday Sunday Monday
Chicago 94 (1911) 97 (2012) 95 (2012)
Rockford 92 (1911) 99 (2012) 93 (2006)
for the 12z tafs...
Southwest winds and VFR conditions are expected today. A lake
breeze will hug the Lake Shore this afternoon, and similar to
yesterday, expecting the lake breeze to reach the terminals early
this evening. An isolated thunderstorm may form along the lake
breeze, but I have low confidence in storm coverage and if storms
will reach the terminals. Kept dry tafs going. East to southeast
winds will be less than 10 kt this evening, and veer to southeast
overnight. Light southwest winds and VFR conditions are expected
late tonight into Sunday.
208 am CDT
South to southwest winds will prevail across the open waters
through the weekend. The weaker flow pattern will favor the
development of onshore flow across the Illinois and Indiana
nearshore waters each day. A weak front drops down the lake later
Monday bringing at least a temporary shift to northeasterly flow.
After that the pattern becomes less certain with the sub tropical
system currently off the Yucatan Peninsula moving into the Gulf
Coast states early next week. The system may interact with a
trough moving across the northern plains. Despite uncertainty in
the pattern, winds look light through much of next week. A push of
stronger northerly winds to around 20 kt is possible late next
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