Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1206 am CDT Friday Aug 22 2014
Short term /tonight/...
issued at 351 PM CDT Thursday Aug 21 2014
Yet another messy forecast for tonight. Currently a shortwave is
exiting Iowa and the associated forcing with it has pushed into
Illinois. Any convection is waning and will be all but gone in an
hour or two. Clouds are clearing fast and the sunny skies are
allowing for clouds to clear or at least lift and we are heating
up...however it may be late enough that highs will remain close to
the going forecast. Not really looking for any storms this early
evening as there will be little to focus on...until the low level
jet increases. However...the latest Storm Prediction Center swody1 still keeps a
slight risk across most of the County Warning Area. Cannot say I agree totally
with it but it is not out of the realm of possibility either.
Their thought is that with clearing will come destabilization and
additional storm development which will sink into far northern
Iowa this evening/tonight. The 4km NSSL does in fact show this
scenario but the hopwrf...hrrr...and NAM 12 fail to materialize
any real convection until later tonight when a shortwave drives a
frontal boundary into US as a low level jet focuses across
My assessment is that later this evening/tonight another shortwave
will cross Iowa flattening the upper ridge a bit coincident with
the low level jet which will focus into southern/southeast Iowa.
There is abundant moisture in place and some weak Theta-E
advection and forcing moves into southern Iowa as well so if all
this comes together...later this evening through a fair portion of
the overnight we should see redevelopment of thunderstorms across
the southern/southeast third or so of the forecast area.
Adding to this will be a surface low over western Minnesota that
will move east southeast tonight as the shortwave pushes across...
dragging a frontal boundary along with it. By 12z the frontal
boundary will lie somewhere near a line from Waterloo to Omaha.
Pretty much kept probability of precipitation confined to along and south of this boundary
with the best probability of precipitation further south along the MO border. North of
this boundary the low level moisture will still reside and
patchy fog will develop. Visibilities could be as low as a mile or
so and it will be possible across a fairly expansive area...enough
to warrant mention in the zones. Patchy fog is possible
elsewhere...south of the boundary as well but it isn't as clear
cut as north of the boundary. Given the possibility of
redevelopment across the north I did carry very low probability of precipitation across
far far north for a couple of hours middle to late evening but kept
higher probability of precipitation south/southeast.
Long term /Friday through Thursday/...
issued at 351 PM CDT Thursday Aug 21 2014
Forecast remained focused on continued hot and humid conditions
and thunderstorm chances over the weekend. Leaned toward the
drier...and thus warmer NAM/European model (ecmwf) through the weekend.
Friday through Sunday...overnight convection will linger across
the south past 12z Friday before peak heating in conjunction with
a weak shortwave riding along the upper ridge by the late
afternoon and evening hours Friday. Much of the forecast area
remains capped through 20z Friday and pulled back the re-develop
until then. Mainly expecting isolated convection late tomorrow
afternoon into the evening before the main surface low and
associated warm front transition across the state. A little better
organized focus by Saturday morning as the warm front bulges into
northern Iowa by 12z Saturday and coincides with a decent slug of
Theta-E advection and qg forcing in this area. Have likely probability of precipitation
12-18z Saturday before convection should move into Minnesota by
the afternoon. Severe potential tomorrow into Saturday are likely
limited with lack of significant shear present.
Tweaked up maximum temperatures on Saturday and Sunday with the warm air advection and good
mixing both days. Heat index values across the central and
southern sections of the forecast area look to top 100 each
afternoon but still below advisory criteria at this time. On
Friday...although southern Iowa heat index values near
criteria...with the morning convection and lingering cloud cover
may inhibit the maximum temperatures and thus low confidence with any
headlines at this time.
Monday through Thursday...large upper level ridge holds in place
over the southeast portion of the country and keeps western to
northern Iowa in the ring of fire. A few additional shortwaves
ride atop the ridge Sunday night through Tuesday night before a
decent cold front pushes across the region Wednesday. Tweaked up
maximum temperatures on Wednesday with good warm air advection ahead of the front.
Otherwise...dry and cooler late next week.
issued at 1205 am CDT Friday Aug 22 2014
System pushing across southern Iowa will bring thunderstorms and rain to southern
sites kdsm/kotm for beginning of period with MVFR and IFR
conditions expected with thunderstorms and rain. Br/fog will develop at remaining
sites near 12z with MVFR or IFR conditions expected. Conditions
will improve quickly after 12z...with winds remaining light and
conditions remaining VFR through remainder of period.