Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1105 am CDT Sat may 25 2013
Discussion...
1048 am CDT
Some changes made earlier to the going forecast. Did further
tighten the gradient in probability of precipitation from west to east across the forecast
area this afternoon into early this evening...while bumped up probability of precipitation in
the far southwest later this afternoon into tonight.
Well-defined short wave on satellite and radar mosaic imagery is
traversing east across Iowa on a 50 knots middle-level jet per rap
analysis. Upper and surface ridging across the immediate area have
helped to stifle the eastward progression of moisture transport and
associated deeper saturation and rain. This will continue to serve
as basically a wall for the eastern forecast area...including
Chicago...through today and likely into the evening. Short term
guidance agrees with observational trends in inching the rain into
the western forecast area as the afternoon progresses. This rain
looks to be light for most of the time into early this evening. Rain
intensity may pick up some later this evening as deeper moisture
convergence works over the area...including even some elevated
instability. There was quite a bit of lightning across Iowa this
morning. While not anticipating anywhere near that much in the
forecast area tonight into Sunday...certainly could see some
scattered thunder.
Temperatures have been slow to climb and certainly look to remain
as such in the western forecast area with thicker clouds and light
rain moving in. Have adjusted down highs a couple degrees toward
local arw guidance which seems to have a good handle on observed
slow climbing trends. Lake influence will keep nearshore areas
plenty cool once again...with highs in the lower to middle 50s. This
is impressively cool for this late in the month. Looking at local
research for downtown Chicago /since 1982/...for the last ten days
of may when the winds are primarily off the lake...the 25th
percentile is 59 degrees and the 10th percentile is 53
degrees...which we will be right around today.
Mtf
//prev discussion...
300 am CDT
Below normal temperatures and periodic chances for showers or
thunderstorms through the weekend will set the stage for a rapid
warming trend for next week.
For the remainder of the night and into Saturday morning...a weak
shortwave overtopping a building ridge over the central Continental U.S. Will
bring increasing cloudiness and a chance for some light showers or
sprinkles...particularly for locations west of the Fox Valley and
the I-55 corridor. Latest radar imagery shows scattered showers
crossing the MS river into northwestern Illinois...but as they move into an
environment noted by dry low level conditions...the showers show a
diminishing trend. However...repeated shower development will
eventually moisten up the lower levels enough for some light precipitation to
reach the ground. Temperatures have already reached a minima across
the area and should begin to slowly rise during the remainder of the
overnight hours as denser cloud cover overspreads the region and
shutting off any further radiative cooling.
For the remainder of Saturday and into Saturday night...will
continue to focus on precipitation chances. The upper level pattern remains
generally unchanged over the past 24-36 hours with a high
amplitude...slowly progressive pattern continuing. The main culprit
is a deep upper low making very low progress lifting northeastward through
New England. This is helping build the upper ridge over the central
Continental U.S. While a series of system move into the Pacific northwest.
While the upper low remains over New England...and the upper ridge
axis remains to the west of the local area...persistent weak cold
advection through deep levels of the atmosphere will help keep
temperatures on the cooler side. Also...at the surface...high
pressure extends from Hudson Bay...through the eastern Great Lakes
to the Tennessee Valley. This has kept northeasterly flow off of Lake
Michigan...with the relatively Cold Lake waters helping to keep
Lakefront temperatures in the 50s. There will be little change
today as temperatures will still remain in the 50s along the Lakefront.
The increasing cloud cover will also help to limit daytime
heating...with highs remaining in the lower to middle 60s. A warm
front is beginning to develop southeastward from a surface low over the Western
Plains. This warm front will increasingly become the focus of
further precipitation development through the remainder of the weekend and
into early next week. The warm front will gradually become more
focused...and extend eastward...through Nebraska...along the Iowa/MO
border to southern Illinois/in by Sunday morning. While the boundary remains
to the south...increasing warm advection and isentropic lift through
the lower levels will help to spread precipitation from the southwest to the
northeast across the County Warning Area through the weekend...though precipitation should
generally be precipitation should begin to become more widespread across
the County Warning Area by Sunday and Sunday night as the warm front slowly lifts
northward. It is a little tough to say if the weekend and the Memorial Day
Holiday will be a total washout...but periods of precipitation will likely
impact much of area. And while thunderstorms will...similarly
likely not be widespread...there will be a persistent chance for
elevated convection in the persistent isentropic lift.
While Memorial Day will likely not be dry...at least temperatures
will be on the increase. By Monday...the upper low will finally
lift northeastward into New Brunswick and the upper ridge will begin to
progress eastward. The models are beginning to loose focus and
continuity in handling the evolution of the upper ridge as there is
little consensus on the timing and strength of additional shortwaves
overtopping the ridge...which will in turn...lead to some
uncertainty to the precipitation forecast. The one general certainty is that
there will be a series of shortwaves tracking across the
region...helping to focus the precipitation in vicinity of the warm front.
By Monday night...the warm front should lift well north of the
area...initiating the rapid warming trend as persistent southwesterly flow
develops at the surface while the upper ridge builds across the
Midwest. However...with a surface ridge extending through the southeastern
Continental U.S....Gulf moisture will be limited through Tuesday. But by
Wednesday...the ridge will retreat to the east...opening up the Gulf
and allow more moisture to stream northward. So...by middle week...expect
that dewpoints should reach into the middle to upper 60s to
accompany the higher temperatures which should reach into the middle
80s. The GFS/European model (ecmwf) are still bringing 850mb temperatures around 18c across
the Midwest...which would imply that temperatures in the upper 80s are not
out of the question given enough sunshine to allow for deep deep
mixing. As usual...the increase in Gulf moisture accompanying the
increased heat will also keep the chances for thunderstorms over the
area. So...the uncertainty in the cloud cover will be the major
question mark right now as to whether the region will see a real
taste of Summer next week.
Krein
&&
Aviation...
//Ord and mdw concerns...updated 16z...
* light northeast to southeast winds.
* A low chance for rain showers late this afternoon and early this
evening. Bulk of the activity expected to be west and south of
Ord/mdw.
Paw
//discussion...updated 12z...
High pressure remains over and east of the area this morning
allowing a light and variable wind flow to be in place along with
a very dry low/middle level airmass. Band of light rain has moved
into north central Illinois and is affecting rfd but the dry air is
eroding this activity as it pushes towards the Chicago metropolitan.
Activity will wind down at rfd in the next few hours and the chi
area terminals could see sprinkles or brief light rain but will
keep Ord/mdw/dpa/gyy dry. Variable northeast winds should become a
bit steadier from the east-southeast over the next few hours but
turn more east-northeast at Ord/mdw this afternoon thanks to lake
influence. Gyy will be further northeast and dpa may see a shift
to east-northeast late this afternoon. Variable winds return this
evening.
Rain showers/thunderstorms and rain will likely linger west of the terminals into the
afternoon before pushing eastward toward the area late this
afternoon. It is not completely clear how far north and east this
activity will go with better chances of it remaining just west and
south of the terminals...except for rfd which has the best chance
for occurrence. Will need to monitor the need to add precipitation
mention at Ord/mdw/dpa/gyy for the late afternoon and early
evening but at this point the potential is too low to include.
Expect VFR ceilings/visibility through the day.
Mdb
//Ord and mdw confidence...updated 16z...
* high confidence in wind trend.
* Medium confidence that rain showers will remain west and south of the
terminals late this afternoon and early this evening.
Paw
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...shra/tsra likely. MVFR possible.
Monday through Friday...mainly VFR...periodic rain showers/thunderstorms and rain especially
Monday and again Friday. Breezy south winds Wednesday and
Thursday.
Mdb
&&
Marine...
242 am CDT
Quiet conditions persist across Lake Michigan with an axis of high
pressure centered just east of the lake. The high will be slow to
move to the east which will keep light and somewhat variable winds
across the lake today...with an onshore component developing along
the shorelines. It will likely take until Monday for a more
persistent southeast flow to set up as low pressure gradually
organizes across the plains and the high shifts to the east. Warmer
air will be moving into the region which will limit mixing and gust
potential but the pressure gradient may be strong enough at times to
allow for periods of 15-25 knots winds by Wednesday. Wind direction
will shift from southeast to south then southwest through the week.
Mdb
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&
$$
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