Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Denver/Boulder Colorado
1011 am MDT Friday may 22 2015

Update...
issued at 1011 am MDT Friday may 22 2015

Upper system is moving out of the state...although a fair amount
of wrap-around moisture is still present across northeast
Colorado. The main changes to the forecast are to lower
the afternoon high temperatures a few degrees due to lingering
clouds and showers. Also added the mention of severe thunderstorms
to the Palmer Divide region where it looks like some middle-level
drying over the next couple hours is going to cause the airmass to
become conditionally unstable. Each of the mesoscale models
continue showing a round to strong showers developing in this
region by early afternoon and then progressing northeastward. If
thunderstorms develop over the Palmer Divide...the brief heavy
rains will be capable of causing flash flooding problems. Am not
sure whether the threat is higher in Elbert or Lincoln County...so
will refrain from issuing any Flash Flood Watch at this time.
Forecasts on the Palmer Divide will mention the chance of severe
with large hail and heavy rain.

Rainfall amounts this morning appear to have remained below one-
half inch at most locations...which does not help the hydrologic
situation...but was light enough that only modest rises on river
levels should result. As the next couple afternoons will see more
convective shower development...localized flooding problems may
become more common.

&&

Short term...(today through tonight )
issued at 300 am MDT Friday may 22 2015

Water vapor imagery showing upper low and back edge of trough
lifting north and northeast across far southern Colorado with bit
of swirl over southeast Colorado. Precipitation has sure taken its time
getting here but radar finally showing an increase in light rain
showers over the plains. Appears heaviest rainfall will be over
the far eastern plains and further west over the West Slope. Given
the low level southeast flow precipitation has weakened some as it comes
off the Palmer Divide. There will be continue weak qg ascent as
the trough moves over northern Colorado than weak to neutral
decent by afternoon. Precipitation rates with the developing showers has
been very light...generally around .04"/hour. Overall quantitative precipitation forecast from
models has also decreased today with overall amounts generally
around a tenth to a half inch on the plains...highest over the far
northeast. Showers gradually decreasing from south to north after
middle morning. Will make some adjustments to current forecast based
on recent trends.

Will keep the Winter Weather Advisory for the mountains until 6 am
as looks like most locations over Summit and Park County are now
getting light snow. Amounts may be a bit high so will taper down a
bit through this morning.

As for this afternoon/evening there looks to be some clearing and
heating initially over the Palmer Divide which would help the
instability cause for thunderstorms today. Surface based convective available potential energy
range from 700 to 1000j/kg from Denver south and southeastward
this afternoon and evening. With surface low over southeast
Colorado appears so sort of convergence boundary will develop over
Douglas and Elbert counties where both rap/hrrr seem to focus
storm development today. Given cape and shear aloft could be a few
severe storms with large hail the main impact. Would expect lower
probability of precipitation and less instability further north and east on the plains
with less heating and instability. Forecast soundings indicate
stratus and patchy fog again later tonight with shallow upslope
flow on the plains. Still could be some lingering showers past
midnight on the plains given continued weak ascent from the next
approaching closed low.

Long term...(saturday through thursday)
issued at 300 am MDT Friday may 22 2015

Synoptically...a large scale blocking pattern over the eastern
Pacific/western Continental U.S. Will continue to dominate our weather over the
next several days. The anomalously deep upper trough with its
closed low spinning over central California at the present time is
still forecast to migrate slowly eastward over the next 48 hours.
Model consensus places the 500 mb low over southern Utah by early
Saturday morning. For your information...earlier GFS runs showed this trough
opening up as it accelerated northeastward across Wyoming during the
day Saturday. It no longer indicates that...at least not until
Sunday.

A dry slot at middle-levels rounding the front side of the trough
should allow for at least a few hours of strong solar heating Sat
morning to quickly destabilize an already moist and marginally
unstable boundary layer east of the mountains as the day progresses moist
subtropical air circulating within the trough will partner with an
influx of low-level moisture carried northward by a strengthening
S-southeasterly boundary layer flow on the plains. The NAM...GFS...sref and
European model (ecmwf) all show a Theta-E ridge surging up over Weld and Logan
counties by early afternoon. These same models also show a surface low
forming in the vicinity of Morgan County about midday and a sharp
deformation axis extending southwest from the surface low to the
foothills in Boulder County...and east across southern Phillips County.
As moisture accumulates along this axis...further destabilization
is anticipated. NAM shows surface based convective available potential energy anywhere from 1900-2700
j/kg and virtually no convective inhibition in the vicinity of the
surface low by 21z. In addition...models show northestern Colorado under the
right entrance region of a departing 90-100kt jet. All this should
result in a few hours of active supercell development...anywhere
from eastern Larimer...Weld...Morgan...nrn Washington...Logan...Sedgwick
and Phillips counties. Large hail will likely be the principle
severe weather risk but with respectable helicity values in the
vicinity of the surface low...can't rule out a brief weak tornado or
two. Farther south...say in and around the Denver metropolitan area...
moisture and instability profiles appear less favorable for deep
convection due to the drying effects of a gusty southerly wind
downsloping off the Palmer Divide. Though scattered probability of precipitation are
still warranted late in the day. In the high country...could see
scattered showers forming over the high ridges as early as late
morning before expanding in coverage together with T-storms
through the afternoon as the upper trough inches closer. Rainfall
amounts are not expected to be all that significant due in part to
fast storm motions. Any snowfall should remain confined to
elevations up around 11000 feet at midday and lowering to around
10000 feet by evening. Saturday temperatures likely to be the warmest
we've seen in a couple weeks with middle/upper 60s at lower
elevations and upper 40s/50s in the high country.

Saturday night and Sunday...airmass is expected to remain relatively
moist as the upper trough makes slow eastward progress across the
region. Although wind shear and instability should be weaker and
temperatures cooler by a few degrees due to cooling aloft and greater
cloud cover. So the threat of severe weather will be much
lower...but will continue the forecast scattered showers and
thunderstorms. With lighter precipitation amounts anticipated...further
runoff into area streams and rivers already running fast and high
should be minor.

Memorial Day through Wednesday will feature showers and T-storms
during the afternoon and evening hours the product of daytime
heating and plenty of residual moisture. Models also show a series
of weak disturbances in westerly flow aloft passing over the area.
Still too early to know with any certainty how these systems will
influence this daily cycle of convection. By Thursday...GFS
indicates a shift to drier and warmer conditions across the
region under a ridge of high pressure. Whereas the European model (ecmwf) shows yet
another shortwave trough bringing clouds...cooler temperatures and
showery precipitation to the forecast area. For now won't stray too far off
from a blend of the guidance. For instance...not all that
confident we'll see upper 70s/lower 80s at lower elevations as
the GFS is indicating late in the period. Will keep temperatures near
seasonal norms for now.

&&

Aviation...(for the tafs through 18z Saturday morning)
issued at 1011 am MDT Friday may 22 2015

Areas of light rain appear to be gradually diminishing as the
upper system moves into southwest Nebraska. Wrap-around moisture
on the northwest side of the departing system will keep showers
going for a few more hours. A little bit of drying at middle-levels
will occur through early in the afternoon...but daytime heating
will help the moist low level airmass to de-stabilize...making
scattered thunderstorms possible through the afternoon. The
strongest storms should be over the Palmer Divide. Any improvement
of ceiling heights into the MVFR or VFR category may be short-
lived as convection begins developing and lifting condensation
levels remain low.

Later tonight...the abundant moisture and weak surface winds may
allow fog and stratus to develop which will bring flight
categories down to IFR after midnight.

&&

Hydrology...
issued at 300 am MDT Friday may 22 2015

Current system only producing light rain showers and don't expect
rainfall rates much above a tenth of an inch per hour. This will
have little or minimal effect on rivers and streams this morning.
Better instability will result in scattered thunderstorms this
afternoon and evening. Could be some brief heavy rain and hail
with storms as precipitable water values are around 3/4 of an inch. This could
result in localized higher stream levels and minor flooding in
locations under a passing storm.

South Platte river levels still increasing across Morgan County
and expect a continued slow rise in river levels today where river
levels will be around flood stage this morning at Weldona. The
South Platte river at Kersey and the Cache la poudre river at
Greeley will remain in flood stage today.

&&

Bou watches/warnings/advisories...
none.
&&

$$

Update...dankers
short term...entrekin
long term...Baker
aviation...dankers
hydrology...entrekin

National Weather Service Glossary of Abbreviations