Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
545 am CST Friday Feb 27 2015
Short term...(today and tonight)
issued at 326 am CST Friday Feb 27 2015
Aloft: a broad trough will remain over the Continental U.S. Through tonight. What
will transpire for the forecast area is best seen at 700 mb. A
shortwave ridge axis will move through this morning with middle-level
winds backing to SW. This will increase clouds from the weak trough
over co/nm. This is already underway.
Surface: high pressure was along the Nebraska/Iowa border and encompassed
much of the central/eastern USA. This high will depart into the eastern USA
through tonight. Return flow develops over the plains.
Today: multi-layered cirrostratus/altocu has been lifting north and
into the forecast area overnight. Expect p/cloudy north of I-80 and
M/cloudy or cloudy S. Forecast soundings at mck/hys indicate eventual
low-level saturation and instability in the ideal snow growth zone
to produce some flurries over the far SW fringe of the forecast area.
So this is new vs the previous forecast.
Continued very cold...but not quite as bad as yesterday.
If you haven't noticed...gri normal high temperatures are in the low 40s.
Tonight: clouds continue to increase and thicken. Continued and
expanded the mention of possible flurries for areas west and S of the
One potential weakness in this forecast is that I may have
underplayed the amount of cloud cover in some areas today-tonight.
The day shift will need to reassess.
Long term...(saturday through thursday)
issued at 326 am CST Friday Feb 27 2015
Guidance suggests quasi-zonal upper tropospheric flow will be
noted over the Central Plains to start the long-term forecast
period at 12z Saturday. An upper tropospheric trough axis is also
expected to be strengthening over the southwestern Continental U.S. On
Saturday which will try to turn our flow more southwesterly...but
the passage of an upper tropospheric short wave over the northern
plains and upper Mississippi Valley Saturday night into Sunday
will help keep this southwesterly flow at Bay and preserve quasi-
zonal flow over our area through the day Sunday. Upper
tropospheric flow should then become more southwesterly Sunday
night through Tuesday as the southwestern Continental U.S. Trough strengthens
and approaches. This trough axis is then expected to clear the
central Continental U.S. By early to middle next week...with northwesterly flow
settling in to finish the forecast period.
Upper tropospheric thermal advection ahead of the southwestern
Continental U.S. Trough axis is still expected to provide enough Omega for
precipitation production across much of the region this weekend.
That said...it is important to note it now appears the primary
axis of upper tropospheric thermal advection...as well as any
enhanced jet dynamics...may actually remain just south/southeast
of our County Warning Area...more over central and eastern Kansas. This is not to
say our area will not observe precipitation this weekend...quite
the contrary. But with the operational GFS...ec...and especially
the NAM now suggesting the strongest band of Omega will remain
just southeast of our County Warning Area...the Main Point being made here is that
notable changes were made to the forecast during this shift.
Starting with the day Saturday...the operational GFS and ec both
suggest ~0.05-0.10" liquid precipitation will be realized across
our extreme southeast...with locations farther northwest receiving
between a trace and a few hundredths...if anything at all. The NAM
on the other hand keeps our area dry for the most part...with the
only exception being three or four counties across our extreme
southeast and even there...it only provides a couple hundredths of
liquid precipitation. Taking a blend of these guidance sets
provides liquid precipitation amounts generally in the 0.01-0.05"
range south of I-80...with maybe 0.01" farther north. This is a
significant decrease in liquid precipitation amounts when compared
to the last few previous shifts...which advertised ~0.20" of
liquid precipitation across our southeast during the day Saturday.
Obviously this will also have significant impacts on snow
amounts...but we will get to that in a bit.
Heading into Saturday night...there remain indications that the
primary axis of Omega could attempt a move back to northwest with
the upper tropospheric jet stream...thus helping promote a round
of heavier precipitation across portions of our area...primarily
our southeast. That said...the operational GFS is really the only
model currently suggesting such a solution...with the NAM and ec
providing much lower precipitation rates. And with the GFS
suggesting near 20 times as much liquid precipitation when
compared to the NAM and ec for Saturday night...it GOES without
saying that the GFS is the odd one out compared to the other sets
of guidance. Nonetheless...opted to consider the GFS
solution...and taking a blend of the NAM...operational GFS and ec
provides Saturday night liquid precipitation amounts of 0.10-0.15"
across our Kansas counties...and in the 0.04-0.10" across our
Nebraska counties. This is a subtle decrease in liquid
precipitation amounts when compared to previous shifts.
Even if the primary axis of Omega does make a move to the
northwest Saturday night...all sets of guidance suggest it will
then make a swift push back to the southeast during the day
Sunday...thus brining an end to precipitation across our entire
County Warning Area by late morning. Taking a blend of model guidance provides
just a few additional hundredths of liquid precipitation across
our area Sunday morning...which again is a significant decrease
compared to the ~0.10" we had across our southeast during the day
Forecast proximity soundings from across the County Warning Area...per the NAM...
operational GFS and ec...all suggest a persistently cold air mass
will continue to support snow as the primary precipitation type
this weekend. Such has been presented in the official forecast. A
blend of the NAM and operational GFS suggests snow- water ratios
will generally range from 15 to 20:1...which is essentially the
same as the previous few forecasts.
Taking the aforementioned liquid precipitation amounts and snow-
water ratios provides 0.5-1.0" of snow across our Kansas counties
during the day Saturday...and between a trace and a half inch for
the most part across our Nebraska counties...although the position
of the Omega axis could push khjh closer to an inch. For Saturday
night...we currently have 1-2 inches generally southeast of a line
from Beaver City to Hastings and Osceola...and less than an inch
farther northwest. And then for the day Sunday...currently have
~0.1" of additional accumulation that morning...with the exception
being north central Kansas east of Highway 281 where an additional
~0.5" is currently forecast. Combining these three periods
provides storm-total snow accumulation of 1-2 across much of the
County Warning Area...with locations southeast of a line from Osborne to Hebron
closer to three inches. Given all this...what seemed like a slam
dunk advisory 12-24 hours ago now appears highly questionable and
thus...no headlines will be issued on this shift so the day shift
can continue to evaluate.
Looking ahead to the workweek next week...guidance continues to
suggest the approach of the upper tropospheric trough axis from
the west/southwest could promote additional precipitation chances
across the area Monday through Wednesday. Supberblend responded by
providing 20-50% probability of precipitation Monday through Wednesday...and these probability of precipitation
were left unchanged but at this time it is far too early to try
and pin down precipitation amounts.
Aviation...(for the 12z kgri/kear tafs through 12z Sat morning)
issued at 545 am CST Friday Feb 27 2015
Today: VFR. For now played the altocu as scattered around 12k feet but
there is potential it could become a ceiling for a time. Light and
variable winds will become S-south-southeast less than 10 kts. Confidence:
high except on the amount of cloud coverage.
Tonight: multi-layered VFR ceilings gradually descend to near 5k feet
and the 06z mav MOS and the consensus of short-term hi-res models
indicate potential for MVFR after 10z at ear. Since models are not
unanimous...went just above MVFR since it is near the end of the
taf period. S-south-southeast winds around 10 kts. Confidence: medium