Technical Forecast Discussion
New York Metro Weather
2:46am December 22nd, 2010
Near Term
Latest regional surface and upper air analysis placed a shortwave over the Great Lakes shifting eastward.
This shortwave was rather strong, but compact, and being ingested into the mean upper level trough
over the Northeast US amidst strong confluence and NW upper level shearing winds. The result for our
area will be eastward advection of high clouds which have made it into Pennsylvania and the MidAtlantic per the latest infrared satellite imagery. This should have a slight effect on overnight lows with
less than ideal radiating conditions. Latest 00z models have adjusted accordingly as have the latest RUC
bufkit data and soundings; and so we went with temperatures a degree or so below the latest OBS.
Regarding the winds...the pressure gradient will loosen up a bit over the next few hours so the winds
should at least temporarily become less of a concern.
Today (Wednesday) will be another cold winters day although temperatures will moderate a bit. Upper
level ridge is forecast to build over the Rockies and eastward towards the Central Plains, but the big
Upper level low over the Northwest Atlantic will drag a shortwave through our area (as mentioned
above) which should keep our area in a modest H5 flow. H5 heights will actually lower to sub 534dm by
18z Wednesday afternoon. The shortwave shouldn't be suffice to warrant PoP in the forecast at this
time, but we will continue to monitor the potential for snow showers especially north and west. Clouds
will be on the increase as mentioned..but not overcast.
Short Term
The main impact of the mid level shortwave passage will be a stronger north/northwest flow and
increasing winds as the pressure gradient gets a bit tighter again. Thursday will be blustery with winds
gusting into the 20 miles per hour range, which will obviously make it feel much colder than it is. We
went a degree or so below MOS. With the winds, overnight temperatures through Friday probably won't
be as cold as NAM/GFS text data indicated as radiating conditions won't be ideal. It will still be plenty
cold.
Mid and high clouds associated with the northwest atlantic upper air troughing could retrograde as far
south as Connecticut, Southeast New York, and New Jersey on Thursday Night. Both the NAM and GFS
seem to increase clouds overnight--so we went a little cloudier than the last package and overnight lows
were adjusted accordingly.
Lots of things happening aloft at this time. Upper level low is forecast to shift northward with an
elongated confluent flow in place over our area through Christmas Eve and Christmas Day. Mid level
ridging is forecast to build over the Central Plains as a shortwave digs into the Southwest states and
eventually ejects eastward towards the MS River Valley. Meanwhile, northern stream heights drop
considerably as the northern stream makes a legitmate attempt to phase over the Ohio Valley. The
sensible weather result is partly cloudy with temperatures in the mid 30's most days...but clouds should
increase by the tail end of Christmas; which should remain dry. Forecast models have trended much
slower with the precipitation nudging towards the region in response to surface low development to our
south and west.
Long Term
Incredibly convoluted and anomalous pattern is forecast to develop in this time period, and we will do
our best to highlight the most recent developments along with our concerns.
The 00z NAM initialized a very strong shortwave trough, associated with the remnants of a Pacific ULL,
about 250 miles off the coast of California. This shortwave is forecast to move rapidly east over the next
24 hours, reaching the coast of California by 00z Thursday. The strong pacific jet and fast and
progressive flow across the CONUS will then force this shortwave eastward. Most forecast guidance was
split as to what happened thereafter, until tonight. Previous runs of the GFS and GEFS means took this
shortwave east very rapidly, bringing the shortwave east of the Texas Panhandle by 06z Friday. The
ECMWF, NAM, and CMC were much slower and more amplified with this shortwave, owing to the
stronger ridging across the Central Plains and building ridge over the West Coast. This solution is now
favored as the GFS has trended about 6 to 10 hours slower with the shortwave on it's latest 00z/22 run.
This results in most guidance agreeing on a strong shortwave, out of the southern jet stream, over the
Texas Panhandle or Oklahoma at 12z Friday. The other main players on the field seem to be relatively
well modeled, with some differences still existing as the model guidance diverges. By 12z Friday, with
the shortwave in TX/OK as previously noted, a relatively high amplitude ridge will be building on the
West coast of the CONUS with the ridge axis through the Western Rockies into West-Central Montana.
Meanwhile, northern stream energy is modeled to be diving southward from Central Canada through
the International Border and into the Dakotas and Northern Plains. Confluent flow over the Northeast
US is expected to be lifting north/northeast as the upper level low moves out of the Northwest Atlantic.
This upper level low will swing north, towards and through the 50/50 lat/lon position, driving lowering
heights through Central Canada and southward.
The result, as advertised on guidance for the past few days and now especially tonight, is the potential
for a major jet stream phase. The GFS misses the phase, sending the surface low from the Southeast
States off the coast of the Carolinas and then out to sea. A closer look at it's H5 depiction reveals its
issue; where the GFS is sending the initial shortwave that was over TX/OK too far south and east-resulting in deamplifying trough heights and a missed phase. The CMC has trended east from it's 12z
solution, but is still enthused with the phase idea. In fact, although precipitation is slightly too far east to
majorly impact the area, the model closes off at H5 south of Long Island, and brings a 970mb surface
low directly over the 40/70 benchmark. The most interesting model so far is definitely the ECMWF,
which has consistently brought the surface low very close to the coast--close enough for major weather
impacts in our area. In fact, tonights 00z/22 ECMWF phases in the Polar Vortex, and traverses the
surface low from 986mb off of OBX, to 972mb northeast of Ocean City, Maryland.
The result of all of this has been tremendous uncertainty. The ECMWF definitely seems to be a far
northwest outlier at this point--but it has been consistent. In any other year or pattern, we would
probably be including most of the ECMWF output in our package. That being said, despite the pattern
amplitude, the pattern's progressive nature suggests we not jump too early on such a solution given the
lack of other global support at this point. On the contrary, the GFS and GEFS mean seem way too flat
and fast--so we did not include much of them into our package either. Instead, we took a blend of the
GGEM and ECMWF from 12z..which brings 40-50% PoP to the coastal areas, and 30-40% PoP to inland
areas at this time. Too early to guess on precipitation types or amounts, but we are not expecting
precipitation type problems given our preferred forecast track. The heaviest QPF is currently forecast to
be in the southeast fringes of our area..tapering off as one goes further north and west.
Obviously, with the uncertainty advertised, things could change considerably. This has the potential to
be a high impact storm system. Current timing favored for impact is Sunday afternoon..through Sunday
Night..and into the first half of Monday. As mentioned..we are taking the middle ground of model
guidance at this point. It's still entirely possible that we see at trend towards either of the most
extremely solutions, which would obviously have significant sensible weather impacts on our area. So
stay tuned. Kept slight chance of PoP in through Tuesday to account for guidance indicating residual low
level moisture as the surface low tracks away from the area. Temperatures through the period should
continue running below normal.
JH