Area forecast discussion
National Weather Service Des Moines Iowa
343 PM CDT Sat may 18 2013
Short term.../tonight /...
issued at 330 PM CDT Sat may 18 2013
Main concerns tonight will be extent of convection across the
forecast area late night and into early morning. Models similar in
timing but very little agreement in areal coverage/placement of
rainfall overnight so confidence low at this time. Synoptic setup
shows approaching h700 shortwave over Panhandle area of Texas/OK
now and simple time of arrival tool shows at current speed will
arrive into SW counties after 07z and into central Iowa by about
10z...favored time for nocturnal convection. This correlates
pretty well with increasing wind fields from the GFS overnight
with h850 40kt jet entering area and sufficient speed shear from
h700/h500 of 40-60kts respectively from 08-12z. Surface boundary looks
to make it as far north as Iowa/MO border by 12z as well. Moisture
is quite high for this event with precipitable waters by 12z approaching +90-95%
level of climatological maximum and near 2 Standard deviations from
normal. Warm cloud depths look fairly impressive with 10-11kft
expected late tonight and a good share of Sunday as well. With
expected short wave and forecast conditions...have some concerns
about locally heavy rainfall and potential for isolated flash
flooding over the west/southwest areas should mesoscale convective system hold together as
it tracks northeast. Current 3 hour gridded flash flood guidance of
+2-3 inches remains pretty high over much of southern 2 tier
counties...but west of i35 and areas north of there have lower
values of 1.75 to 2 inches in general. With high precipitable waters and
significant warm cloud depths combined with short wave we cloud
see isolated rainfall amounts of 1 to 2+ inches over the southwest
to central counties by 12z...within track of mesoscale convective system. The challenge
remains however...in that the 4 km WRF and hrrr will need to watch
this through evening to see how situation evolves. Lows overnight
will be quite mild again with increasing moisture putting a cap on
mins in the lower to middle 60s. Some concern remains about potential
for severe overnight...but if any occurs will be mainly for large
hail over the west.
Long term.../Sunday through Saturday/...
issued at 330 PM CDT Sat may 18 2013
Main forecast concern remains focused on active weather pattern
developing over the next several days. Models in good agreement
through early in the week with cutoff low lingering through
Thursday. Leaned a blend of the nam12 and arw-WRF for timing and
trends tomorrow into tomorrow night...then a GFS/European model (ecmwf) through
midweek.
Sunday into Monday...strong 500mb trough digs into the Central
Plains tonight and slowly make its was into the Midwest by tomorrow.
First shortwave that may spawn thunderstorms overnight tonight
lingers into northern Iowa through Sunday morning and diminish as
the low level jet decreases. Decreased probability of precipitation across the south considerably
until the late afternoon before redevelopment by the afternoon
before next shortwave ejects across the Central Plains and into
MO/Iowa by 00z Monday.
Destabilization occurs late Sunday morning and into the afternoon
as the 0-3km ml cape increases to 800 to near 2000 j/kg over
central and southern Iowa by 00z Monday with break between
shortwaves. Surface based cape increases to over 2500 j/kg by the
same time and location. Low level shear more favorable for
tornadic development across far southwest Iowa where 0-1km shear
increase to 20-25 knots...more in the 10 to 15 knots range further
north. Deep layer shear favorable for long lived storms late
tomorrow afternoon into the evening hours. Kept mention of severe
weather for southern portions of the forecast area Sunday late
afternoon and into the evening. In addition to severe
potential...heavy rain looks imminent as well with warm layer
cloud depths ranging around 11000 feet and precipitable water
values in the 1.5 to 1.75 inch range. Corfidi vectors indicating
training storms tomorrow night leading to flash flooding
potential. Storms look to push into northern Iowa early Monday
morning and lowered probability of precipitation in central and southern Iowa.
Tuesday through Saturday...upper low becomes cutoff and lingers
over the region before finally moves east by Thursday morning.
Severe weather chances look minimal...but additional periods of rain
are likely into Thursday. Temperatures look to be near or slightly
below normal by Wednesday and through late in the week.
&&
Aviation...18/18z
issued at 1246 PM CDT Sat may 18 2013
Overnight concerns regarding convection and overall coverage.
Models suggesting mesoscale convective system development near/in area by 10-15z Sunday
but placement somewhat challenging for now. Have considered warm
air advection and nose of increasing southerly jet with warm
frontal placement...all pointing to development affecting either
kdsm or kfod greater than areas east/north. Thus...for now have
increased likelihood of thunderstorms and rain at those two sites for early morning
hours. Winds will likely remain just under 12kt overnight with
increasing southerly flow after 04z. With overnight convection...
begin to expect the mesoscale convective system to diminish toward the end of period as
jet weakens after 12z. Next round of convection more likely late
afternoon or after 20/00z as shortwave approaches from west.
&&
Dmx watches/warnings/advisories...none.
&&
$$
Short term...rev
long term...podrazik
aviation...rev
