Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
653 am CDT Thursday Jul 31 2014
issued at 653 am CDT Thursday Jul 31 2014
The hourly forecast has been updated through 2 PM. Other than short-term
adjustments to temps/dwpts/winds...the main change was to remove
probability of precipitation for most of the morning.
Forecast is evolving as anticipated. Thick cirrostratus is moving through
with patches of beefy altocu /accas/ indicating the middle-level
Short term...(today and tonight)
issued at 353 am CDT Thursday Jul 31 2014
..after another pleasant stretch of cooler temperatures we return to
normal today as we close out a cooler and fairly dry July for
most of the region...
Aloft: no change. Northwest flow will continue to dominate. There is a very
small trough over South Dakota /best seen at 700 mb/. This trough will continue
dropping S...crossing Nebraska today and tonight.
Surface: high pressure continues to encompass most of the eastern USA.
Embedded in this high was a very weak low over South Dakota...associated
with the middle-level trough. This low will continue moving S...crossing Nebraska
along with its middle-level forcing...to be located near Concordia Kansas
at daybreak tomorrow.
Looking at visible satellite and radar loops from Wednesday afternoon...a
few isolated/small showers developed around the southern fringe of the South Dakota
low. There was only one cloud-to-ground lightning strike. 00z unr/abr soundings
show moisture is lacking. The observed abr sounding is very
similar to what the NAM BUFKIT showed 24 hours ago...with
SBCAPE/MLCAPE only 340 j/kg with a very thin cape profile. So the
sref forecast of 750-1000 j/kg looks way overdone. I originally was
not planning on putting thunder in the forecast...but between 1 am-3
am cloud tops have been cooling over eastern South Dakota with new showers
forming. There have been a couple strikes. So thunder has been
maintained from the 4 PM forecast.
The Sample tool shows cloud top temperatures around -30c at 330 am which
translates to tops of 27k feet. Weak updrafts will be the limiter of
The forecast of showers/thunderstorms will make the potential sound worse than
it really is. Would prefer the forecast worded as slight chance of
showers and possibly a rumble of T. Mitchell South Dakota /mhe/ did measure
03" between 2-3 am. If isolated showers are developing overnight
there should be no problem popping a few with daytime heating.
The NAM continues the Leader on this shower potential but the 00z Gem/
GFS have come around with their quantitative precipitation forecast. The 12z/00z ec runs dump
some light quantitative precipitation forecast as well...but only NE of the forecast area.
Today: large patches of thick cirrostratus will depart from west-east
this morning. Daytime heating will result in widespread cumulus across
S-central Nebraska...some of which will end up as embryos for shower
The 4 km NAM nests are handling the overnight shower activity very
well vs reality at 3 am. They suggest that while an isolated shower
might develop before noon...the main initiation will be in the 2-4
PM time frame...possibly in a broken arc of showers that tracks southeast
through early evening. Morning probability of precipitation may be scaled back or removed
altogether later this morning.
Dewpoints really dropped yesterday afternoon S of I-80. So I used
a 50-50 blend of the NAM 2m dewpoints and mav MOS /for 21z-03z/ which
have been verifying the best over the last week. 30f surface dewpoint
spreads and cloud bases near 7k feet could result in gusty showers.
Would not be surprised to see an isolated gust or two up to 40
miles per hour.
Forecast confidence: high
Tonight: partly-mostly cloudy in the evening as showers fade with
loss of heating. Cant entirely rule out an isolated shower surviving
until midnight S and east of the Tri-Cities. Gradual clearing from
northwest-southeast as diurnal clouds dissipate and leftover cloud debris
departs to the southeast.
Forecast confidence: medium
Used bias corrected consensus of model 2m temperatures for both today and
Probability of precipitation were an even blend of the 18z/00z NAM and 15z/21z sref...cross
checked with the hi-res NAM nests. Very short-term probability of precipitation will need
to be increased as radar trends unfold later today.
Long term...(friday through wednesday)
issued at 344 am CDT Thursday Jul 31 2014
Guidance suggests northwesterly upper tropospheric flow will be
noted over the Central Plains to start the long term forecast
period...and will persist through Sunday or Monday of next week.
By early next week...the western Continental U.S. Upper tropospheric ridge is
expected to build into the central Continental U.S....allowing for weakening
upper tropospheric flow...with a more zonal flow then expected by
the end of the long term forecast period as an upper tropospheric
short wave moves south/southeast from southern Canada.
Although most model output suggest an overall lack in Omega will
present dry conditions to the area until at least the middle part
of next week...the NAM does suggest precipitation production could
be realized over portions of the County Warning Area late Friday night into
Saturday as a subtle upper tropospheric perturbation clears the
area within the mean northwesterly flow. Since the NAM is the only
model to suggest precipitation will be realized over our
area...opted to go ahead with a dry forecast Friday through Monday
and wait to see if other models join the NAM solution. Heading
into Tuesday and Wednesday...guidance suggests Omega will increase
across the region as the aforementioned upper tropospheric short
wave approaches from the north/northwest. Allblend responded by
providing 20-40% probability of precipitation to much of the County Warning Area Tuesday through Wednesday
and these probability of precipitation were left unchanged for the most part.
Little if any change in the lower tropospheric air mass should
promote similar temperature readings across the area from day to
day through the long term forecast period...with afternoon highs
in the 80s to near 90 and overnight lows in the 60s expected for
the most part.
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 653 am CDT Thursday Jul 31 2014
Today: multi-layered VFR clouds will diminish this morning and
give way to vigorous altocu and possibly cumulonimbus development this
afternoon around 7k feet. An isolated probably VFR shower could
threaten the terminals and this will result in wild/erratic gusts
up to 35 kts. Otherwise mainly SW winds 10 kts or less.
Tonight: VFR. Any shower activity will end leaving middle-level cloud
debris between 7-10k feet that will gradually diminish. Variable
winds under 10 kts. Confidence: medium