Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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000
FXUS63 KDVN 191755
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1255 PM CDT Wed Apr 19 2017

.UPDATE...
Issued at 1255 PM CDT Wed Apr 19 2017

At noon, the front had become nearly stationary from near Peoria
west through Burlington and along the IA/MO border through the
southeast corner of Nebraska to low pressure over far north
central KS. North of the boundary, temperatures were in the 50s to
low 60s over much of the forecast area, while to the south,
thinning CI was allowing surface temperatures to warm well into
the 70s. The remnants of the convective complex and MCV that
moved through the area this morning was over north central IL,
while area radars showed another area of mainly light showers and
a separate MCV lifting northeast through east central IA.

SPC`s late morning update shifted the both the enhanced risk and
slight risk westward, as the next round of thunderstorms is likely
to focus close to the advancing low over KS and upper level
shortwave just now moving out of WY into SD and western NE. Out in
advance, while the warm front is expected to gradually lift
northward through the forecast area, an elevated warm layer is
likely to cap any development during the day along the boundary.
Tonight, the best shear is likely to stay focused close to the
warm front and surface low tracking into northeast IA, with
showers and possible thunderstorms with the trailing cold frontal
passage late tonight over the forecast area. 12z models suggest
the bulk of overnight convection will be focused along the low
track and possibly into the northwest portions of the forecast
area, with the GFS the most heavy handed with QPF, while the NAM
would keep most of the area dry. Backing this latter approach,
convective allowing models are mainly dry with convection passing
to the north and south of the forecast area overnight and limited
weak development with the cold front. The HRRR suggests a few
cells possible this evening along the warm front bisecting the
area.

Thus, there is low confidence on the potential for storms and severe
weather tonight and evolution of the system will continue to be
watched. Pops have been lowered to chance category for overnight.

&&

.SYNOPSIS...
Issued at 342 AM CDT Wed Apr 19 2017

07z MSAS analysis places a cold front from eastern WI near Green
Bay into IA roughly along a Dubuque to Cedar Rapids line, and
then extends southwestward through northern KS where it turns
stationary ahead of a deepening cyclone emerging from the Lee of
the Rockies. Strongest low level jet and moisture transport is
aimed into the western flank of the frontal zone aided by
approaching mid level shortwave. This has led to more robust
coverage of showers and storms over portions of NE/northeast
KS/northwest MO/southwest IA. Weaker branch of jet and elevated
moisture transport is veered into our portion of the boundary,
thus only isolated showers are being found along and ahead of the
front for now. This aforementioned shortwave will lift ENE today
and bring showers and some storms to the region. A stronger
shortwave just crossing the Rockies will follow in behind and
organize into a deeper shortwave trough while lifting into the
Upper Midwest tonight. Attendant surface low will follow along
and lift the front back northward through the cwa as a warm front
thrusting cwa in unstable warm sector with a risk of severe storms
this evening into the overnight ahead of an approaching cold front.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 342 AM CDT Wed Apr 19 2017

Cold front looks to stall near the IA/MO border into west central IL
this AM before beginning to return nwd as a warm front this afternoon.
However, high pressure shifting through the Great Lakes will turn the
low level flow from the ENE over Lake MI and send another back door
cold front out way later this afternoon into evening. This will serve
to impede northward advancement of warm front to most likely between
I-80 and Hwy 34 through early evening. Will see shower and storm chances
greatest this AM into early afternoon from sw to ne attendant to passing
weak lead upper impulse. This activity looks to remain elevated and not
pose much risk aside from lightning, locally heavy rain and perhaps small
hail with strongest cores with MUCAPE around 500 J/kg and mid level lapse
rates around 7C/km. Once this impulse passes early this afternoon, then
a lack of forcing and an advecting EML on swly flow aloft look to largely
suppress any development rest of this afternoon and CAMS in pretty good
agreement on this scenario.

Late this afternoon and evening, as surface low pressure lifts northeast
into the Upper Midwest attendant to upper level shortwave forcing will
increase over the region. This should serve to weaken the EML and
allow convection to develop. Initial development is mostly likely to
occur to our west near the low and triple point. Some of these storms
may quickly become supercells given magnitude of shear, and likely
scenario is for these storms to congeal into a line with potential
for bowing segments while heading eastward through the evening.
For our area the severe weather threat will most likely to be attendant
to this line of storms, with damaging winds primary threat likely
arriving into eastern IA and northeast MO mid to late evening. If
storms can form on the warm front out ahead of this approaching line,
then will likely be supercells with threat being large hail further
north of warm front, while all modes would be possible along the warm
front including possibility of tornadoes with backed low level
flow increasing 0-1km SRH. The warm front is generally being
suggested from northeast IA through the Quad Cities to near
Princeton IL much of this evening before lifting north, and
along just n of this front would be most favored for tornadic
potential. Aside from the severe risk tonight, there will also be
an attendant threat for heavy rain with PWATs increasing to over
1.3 inches. Can`t rule out a threat for flash flooding with the
most intense cells especially over portions of northeast and east
central IA which is saturated from recent rains. However,
sufficient storm motion may preclude or limit threat and may aid
in keeping coverage of higher amounts to more localized, as CAMs
generally show some weakening of the convection while approaching
the Mississippi River late this evening though not likely to
nearly complete dissipation being suggested by a couple of CAMs.

Bottom line, threat for severe storms exists late this afternoon, and
especially tonight (5p-4a). Main threats will be damaging winds and
large hail but tornadoes are possible with any interaction with
warm front and also possibly embedded in any lines with favorable
shear-vector orientation. Greatest severe threat is likely west of
the Mississippi River. Locally heavy rain and isolated flash flooding
also possible.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 342 AM CDT Wed Apr 19 2017

Below normal temperatures to end the work week and over the weekend.

Thursday morning, lingering showers and possibly a few thunderstorms
will be seen across the southeast half of the area as the cold front
moves east of the area.

Quiet and dry conditions will be seen Thursday afternoon through
Friday as cool Canadian high pressure moves through the Midwest.

Friday night the next storm system will begin affecting the Midwest.
The strength of the high over the Great Lakes will be the key as to
whether or not there will be any precipitation across the area. The
current model consensus has slight chance to chance pops Friday
night south of an Ottumwa to Galesburg line.

Saturday on...

On Saturday, several model solutions keep the entire area dry with a
stronger high over the Great Lakes. As a result, the model consensus
has slight chance pops south of an Ottumwa to Macomb line during the
morning and dry conditions in the afternoon. Temperatures will
average below normal.

Saturday night through Tuesday the model consensus has mainly dry
conditions as high pressure moves toward the east coast and
southerly return flow develops across the Midwest. Temperatures
should average a little above normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 1255 PM CDT Wed Apr 19 2017

MVFR to IFR conditions will be likely this afternoon in the cool,
moist airmass north of a warm front along the IA/MO border into
central IL. As this front lifts north, conditions should improve
to MVFR to VFR overnight with winds veering from the east-
southeast to south. There is a low potential for thunderstorms
this afternoon and overnight as a wave of low pressure lifts
through the region. PROB30 groups have been used to cover the
most likely periods for showers and thunderstorms at the
terminals. A cold front moving through late tonight toward
morning will veer winds to westerly with low confidence MVFR
stratus possibly lingering through much of Thursday morning.

&&

.HYDROLOGY...
Issued at 1059 AM CDT Wed Apr 19 2017

On the mainstem Mississippi, morning trends and latest flood
guidance now indicate minor flooding is likely at Burlington and
have issued a flood warning at that point. Gladstone will also
rise above flood stage today with minor flooding continuing into
early next week. Keithsburg is forecast to rise near flood stage
Thursday night, and with limited confidence, have issued a flood
watch. New Boston may reach flood stage, but not until Sunday
night, so will hold off on a flood watch there until confidence
increases. From Muscatine and points upstream, in bank rises are
forecast into next week.

On the tributaries, latest trends suggest Wapello will stay below
flood stage and have cancelled the watch. Elsewhere, flood
warnings remain in effect at Joslin, Marengo, and DeWitt.

The heaviest rainfall overnight into this morning has occurred
south of Interstate 80, with isolated swaths up to 1.5 inches.
These amounts are not expected to have a significant impact on
rivers. Looking ahead, the overall threat for organized
additional heavy rain tonight looks low. Thus, the risk of more
significant and widespread river flooding this weekend into next
week is lessening. Will still be monitoring closely tonight. If
heavy rain was to occur, areas north of I-80 would be favored.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

UPDATE...Sheets
SYNOPSIS...McClure
SHORT TERM...McClure
LONG TERM...08
AVIATION...Sheets
HYDROLOGY...RP Kinney



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