Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
257 am CDT Friday Jul 31 2015
226 PM CDT
Forecast period will remain relatively quiet...with dry
conditions expected. Increasing westerly winds at this time will
likely reach their peak over the next couple of hours with a
diminishing trend then expected into the early evening. A few cumulus
have developed this afternoon but these will quickly erode with
clear skies expected once again tonight. Guidance is hinting at
some passing middle/high level cloud cover late tonight into Friday
morning. However...feel it may be a bit aggressive and dont think
it will amount to much beyond a window of partly cloudy skies.
Sunny skies will help temperatures across the area to be somewhat of a
mirror of today...with offshore flow keeping any lake cooling
limited across portions of Northwest Indiana. Dewpoints will try
to increase throughout the day but should be limited to the lower
60s before daytime mixing allows drier air to mix down...with this
lower dewpoint air also limiting heat indices. Was a little
concerned that previous runs of guidance were not quite handling
precipitation potential Friday afternoon quite well but 12z guidance
today has continued a similar trend...keeping any development out
ahead of approaching trough/boundary outside of the County Warning Area. With low
to middle level flow already west to northwest on Friday...low/mid
level convergence will remain rather weak. Also...support aloft
appears to be lacking. So have backed away on thunderstorm chances
Friday afternoon for the County Warning Area but if any brief/isolated development
were to occur...it would be along the Ford/Iroquois to Benton
251 am CDT
Saturday night through Thursday...
500mb trough axis will be pivoting east across the eastern Great
Lakes Sat...while ridging persist across the western Continental U.S.. oper
solutions continue to feature a middle-level wave developing Sat afternoon
across the plains...and quickly lifting east and reaching the
forecast area late Sat night. Instability will be on the increase as
the wave slides overhead...however timing is less favorable for
well organized/developed thunderstorms. Some of the guidance
members suggest arrival of the wave will be after 6z sun.
The 500mb trough axis continues to send a few weak lobes of
vorticity south across the upper Midwest sun/Mon. Marginal
instability will linger sun/Mon...so will maintain the mention of
showers/thunderstorms. Although at this time expect the best chance
for thunderstorms would occur in more of a diurnal component...in
the afternoon/evening timeframe both days.
Ensembles then trend back towards the western ridge
developing...helping to bring a slightly stronger trough back across
the Great Lakes region towards mid-week. Minimal spread continues to
be advertised amongst most ensemble members...suggesting a
relatively high confidence. A frontal boundary will push across the
region Tuesday...with unseasonably cooler air steadily advecting south
with a thermal trough setting up over the Great Lakes region. This
will transition temperatures from the middle/upper 80s sun/Mon...to around 80
Tuesday...and then middle/upper 70s for the remainder of the extended
periods. Chances for showers and isolated thunder looks minimal
beyond Tuesday...and could end up remaining dry as some guidance
suggests weak anti-cyclonic flow lingering overhead.
//Ord and mdw concerns...updated 06z...
* west winds mainly 7-10kt overnight...increasing middle-morning with
gusts between 20-24kt.
High pressure remains overhead...allowing VFR conds to prevail
through entire taf period. Winds will remain westerly with better
mixing middle/late morning increasing winds to 12-15kt and gusts
20-24kt. Gusts diminish after sunset...but remain west. Only a few
clouds midday with bases around 7-8kft above ground level. Otherwise a thin cirrus
//Ord and mdw confidence...updated 06z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
* Saturday...VFR. West winds. Chance thunderstorms and rain overnight.
* Sunday...chance of thunderstorms and rain. Southwest winds.
* Monday...chance of thunderstorms and rain. Northwest winds.
* Tuesday...dry/VFR. Northeast winds.
* Wednesday...chance of thunderstorms and rain. Easterly winds.
255 am CDT
The gradient tightens up again today...with west/northwest winds
creating some gusty conds mainly for the southern half. This will
create hazardous conditions for small craft through this
afternoon with 20-25kt gusts...then after sunset the gradient
diminishes allowing winds to become less gusty. With high pressure
drifting east across the Ohio Valley Saturday...winds will be
slowly turning southwesterly. The gradient tightens late in the
weekend as another area of low pressure approaches Lake Michigan
from the west. Winds may become gusty Sun afternoon. A stronger
low and frontal boundary will arrive Monday night into
Tuesday...with winds turning northwest to north.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743...10 am
Friday to 7 PM Friday.
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