Next week Prognostications (President's Day)

[Planetsnow]

MEDIUM RANGE OUTLOOK
(weather trends through the next 6 to 10 days)

By Meteorologist Larry Cosgrove

With the clock ticking away (no, I am not implying that winter is winding down or that "meteorological winter" is anything but a data time stop), many of you are probably wondering about any potential snow or ice hazards on the horizon. A leading candidate would appear to be the storm now taking shape west of CA, since some (not all) of the numerical models and their ensemble members have suggested that this feature could bring widespread frozen precipitation from the Great Plains to the Northeast during the February 19-22 time frame. That is all well and good, but keep in mind that some of these same computer outlooks also told us that Boston MA was a sure bet to get two feet of snow last week. So we should first look at the overall setup of the atmosphere and see if some of the requirements for a big cold outbreak and huge snowfall machine are in place.

North Atlantic Oscillation

The general rule for using the NAO is simple: is the core of a blocking anticyclone within coordinates 85 W Longitude, 45 N Latitude, 70 N Latitude, 40 W Longitude, and does that center measure more than 534dcm at the 500MB level? Despite comments that I have heard that the NAO is or will be "ragingly" negative, the fact remains that for the past four run times only one of the highly used NWP schemes shows a true block within the defined oscillation coverage zone. At 0z Feb 13, it was the GFS version, while prior to that the GGEM equation showed what amounted to a Rex signature near Iceland. Bottom line: it is not a sure thing that the North Atlantic Oscillation will be in a negative phase during the longer term. I would rate odds on a block formation in the above listed geographic area to be no better than 1 in 4.

Grand Banks Vortex

Typically, a storm system rising from lower latitudes out of the Intermountain Region needs a cold air field to draw from if copious amounts of snow and ice are to be produced (this is especially the case in the Northeast). The cA values are held in place by two interactions: a surface high in ONT, QC, or LBR and a closed 500MB low just east of NWF with core heights of less than 535dcm. All of the most recent computer forecasts show a negative height anomaly in extreme E QC (near the mouth of the St. Lawrence River) by 168 hours. However, NO model shows a closed circulation in the vicinity of the Grand Banks (the vaunted 50/50 low). It is entirely possible that a vortex will take shape beyond Day 7 IF the European scheme verifies, and a solid majority of the NWP versions show what appears to be a cold anticyclone over the Laurentian Shield into S LBR. So the parameter here is favorable, although I would caution not in a classic sense.

Pacific North America or Positive Negative Anomaly

If you want to see a widespread surge of Arctic air into the lower 48 states, the ideal scenario entails having a strong ridge in the "PNA" position: 500MB positive height anomaly boxed within coordinates 45 N Latitude, 100 W Longitude, 155 W Longitude, 70 N Latitude. An anticyclonic circulation located within that bounded region would produce northerly flow to the right of the Continental Divide, and if a concurrent NAO-styled block were in place the cAk values would have a chance to reach even the southern tier states. If you review NWP data over the western third of North America, you notice that a Rex block is located over the AK Panhandle/coastal BC early in the medium range but there is noticeable weakening on ALL of the numerical outlooks by 168 hours. So while a moderate/weak +PNA measure seems to exist in the longer term, implications are that the southern third of the nation will escape any intrusion of Arctic air.

Presence of Storm Energy In Northern And Southern Streams Phasing

The "textbook" massive winter storm formation occurs when THREE jet streams (Arctic, Polar, and Subtropical) phase together around one strong disturbance emerging from the Intermountain Region. There is, quite obviously, a well-developed low lurking over the Eastern Pacific Ocean, and most of the operational equations predict that this system will have an impact on a swath of territory from CA and OR into the Eastern Seaboard. The question then arises: will the low be its own singular entity or be joined by other impulses? Since I see no 'warm' component (only two wind/height field bandings show up on virtually all of the longer term forecast depictions), any additional energy would have to come from the higher latitudes. The 0z Feb 13 ECMWF shows a fairly vigorous shortwave in BC/AB which MIGHT interact with the West Coast cyclone; the same time GGEM shows the CA feature ejecting as a smaller scale, although intense, hybrid-type low that could have serious frozen type impacts across the Great Plains, Midwest, and Northeast. The concurrent runs of the GFS and UKMET indicate either a fragmented (two or three small disturbances in a series) or stalled (stuck over CA of the Southwest through February 19 scenario. So upon examination, we have NO model cohesion on how the western U.S. energy will track or evolve in the days ahead! But we know that a king-sized event (like Jan 7-8 1996, or February 2003) is not visible on model data, at least at this point in time.

What can we draw from the above review of the medium range?

- Bitter cold air will dominate much of Canada but solidly impact only the states above 40 N latitude. The southern tier of states will generally dodge the bullet, so to speak, of Arctic values.

- The West Coast is the only place in the U.S. assured of a long-term precipitation event. At least three days this week will feature either stratiform or convective rainfall in large quantities in S OR....CA....AZ....NV. So mudslides are a concern again for the Golden State, and flooding might redevelop in the Desert Regions.

- There is potential for an important winter storm to take shape next weekend into February 21-22. But this is NOT a sure bet, and in all truth, the Canadian model idea of a "singular stream, ride the cA boundary" type of storm makes as much sense as the implied (not actually shown) phasing of an Arctic shortwave with the CA storm. When and if a low organizes, there will be an impact from an upper air pattern that would hold cold air in place across the Great Lakes and Northeast. So whatever does transpire in the way of precipitation, odds seem to favor snow and ice, at least above the Ohio River and Mason-Dixon line.