Slightly revised 1pm
After a month-long stretch of almost uninterrupted below average temperatures and snow, with 42.1″ of snow having accumulated in Central Park between 1/21 and 2/18, warmth will temporarily return for the next few days, peaking tomorrow with highs in the upper 40s and 50s, followed by a line of heavy rain and thunderstorms. This warmth will prove to be just temporary, however; by next week, an anomalously cold frigid air mass not seen since January will crash southwards, settling just north of the US and providing a return of well below average temps and ultimately more snow (Image credit: NCEP MAG).
Today – Tomorrow: Brief Warmth, Then Rain/Thunderstorms
Storm Synopsis: Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized GFS 500 millibar heights and vorticity from NCEP MAG; both are valid at 12z (7am). A highly complex and dynamic low pressure system is currently developing near Kansas, with a minimum pressure near 992mb. The warm front can be seen extending to its east into the southern Ohio Valley region; this southerly flow along with a strong low level jet is advecting a warm and moist air mass from the Gulf of Mexico, with dew points as high as the 60s to its south. With the aid of the warm and moist air advection, a complex of heavy thunderstorms developed over Iowa last night and has expanded into Chicago and northern Indiana, which will continue to progress to the ENE ahead of the warm front later today into the evening hours, ultimately producing a narrow swath of moderate-heavy snow and freezing rain in the northeast US.
12z run of the high resolution 4k NAM at hour 12, at 0z tonight (7pm), depicting developing blizzard conditions in Wisconsin, severe thunderstorms from Kentucky and further southwest, and a narrow band of heavy snow in central NY state (Image credit: NCEP MAG).
As the two shortwaves near North Dakota and Texas, which can be seen in the 500mb millibar map, phase tonight near Wisconsin with a closed upper level low developing, the surface low pressure will continue to rapidly intensify as it tracks to the northeast, reaching a minimum pressure as low as 976mb while over Wisconsin. As the strengthening low level jet continues to pull in warm and moist air into the system, along with increasing instability in the warm sector contrasting with a surge of dry, cooler air behind the cold front, a line of strong to severe thunderstorms is expected to develop late this afternoon in the western Ohio Valley region, especially focusing near Kentucky and Tennessee, where SPC has issued a moderate risk of severe weather. In the northwestern quadrant of the low pressure, meanwhile, the rapid deepening of the low and tightening pressure gradient will support blizzard conditions over Wisconsin tonight with over a foot of snow expected.
Impacts in the NE US: The storm will affect the region in two waves; the first will contain wintry precipitation tonight and focus north of the area, while the second will involve a more spring-like situation in the region with a brief spike of warmth and a line of thunderstorms. The aforementioned complex of thunderstorms in the northern Ohio Valley will continue to expand ENE, ultimately reaching the northeast US with enough cold air remaining in place to provide support for a possible narrow band of moderate-heavy wet snow between approximately Albany, NY and the Adirondacks extending eastward. Following the passage of this band, freezing rain will develop in the higher elevations of the northeast with over 0.10 inch of ice expected. The NYC area will mostly remain dry today with increasing clouds today and highs in the mid-upper 40s for most, falling into the 35-40 degree range tonight as the warm front slowly advances northeast but fails to reach the area until at least the morning hours on Friday. With warmer air entering the region aloft as temperatures near the surface remain relatively cooler, areas of drizzle and light rain showers will develop across the area tonight, along with areas of fog and low visibility.
The main highlight for the area will be tomorrow as the cold front enters the region and the warm front pushes through, allowing for a brief spike in temperatures into the low-mid 50s in the immediate NYC area into parts of Long Island and possibly southern CT, while interior northwest locations likely struggle to warm above the low 40s. While the best forcing will shift north of the region, marginal instability ahead of the cold front along with a strengthening low level jet will aid in a line of heavy rain quickly tracking east through the region reaching locations from NYC and further north/west around 1-3pm and further east by 3-5pm; some thunder is possible along with this line, with the highest risk of more frequent lightning and thunder to the south of the area. This line is depicted to the left from the latest 12z run of the 4k NAM. Strong wind gusts over 40 mph are possible especially with the heavier showers, which would be capable of mixing down some of the stronger winds aloft closer to the surface. The mild temperatures and rain will especially be capable of partially melting the snow pack, leading to a risk of some flooding with the snow melt. The line will remain fairly short lasting, with up to 4 hours of rain expected and totals generally less than 1/2 inch, which will be followed by temperatures crashing back into the 30s for most locations other than Long Island and parts of CT by the evening.
Weekend Outlook: Mild And Dry
This weekend’s conditions, while rather inactive, are a rarity for a winter dominated by cold and snow with occasional brief spikes of warmth; mostly sunny skies and relatively mild temperatures. As today’s Midwest blizzard becomes a strong upper level low over the southern Hudson Bay, the core of the cold air mass will be pulled in to the north as well, with a southerly flow in the mid-upper levels allowing the colder temperatures to only slowly filter into the region, leaving relatively mild temperatures in the region in the meantime. Highs on Saturday are expected to peak in the upper 40s to low 50s, with highs on Sunday in the low to mid 40s.
Next Week: Winter Returns With Force
6z GFS run at hour 168, depicting the meridional flow with a highly amplified ridge over western Canada and a strong piece of the polar vortex near southern Canada (image credit: NCEP MAG).
The period of warmer temperatures this week and weekend will remain only a taste of spring, as winter will make a forceful return into the region with widespread below to well below average temperatures expected across the eastern half of the US. Earlier last week, a temporary relaxation began in the Pacific pattern, which until that point had featured a persistent ridge and/or block in the north central or NE Pacific, leading to a strong -EPO index which has dominated the North American pattern more so than the lack of a -NAO or blocking in the North Atlantic, which often would be detrimental to sustained cold and snow. An upper level low remained stationed near to just south of Alaska over the last week, which aided in the current moderation of cold, but will quickly fall apart as strong ridging rebuilds over the northeast Pacific, detaching into a strong block over Alaska, along with the aid of tropical forcing as the MJO propogates eastward into phases 7-8, which often correlate to a higher probability of below normal temperatures in the eastern US. With the highly amplified ridging over the northern Pacific into Alaska establishing a strong northerly flow over western Canada, a strong piece of the polar vortex, along with the core of the coldest air mass in the northern hemisphere, will crash southward through Canada, ultimately settling just north of the US/Canada border by mid-late next week accompanied by a frigid air mass with a widespread core of 850mb temperatures well below -30C, a setup not seen since January, and bringing about the return of cold and ultimately more snow.
Monday – Wednesday: The gradual cool down will continue into the first half of next week as a series of shortwave troughs race through the region, with temperatures on Monday returning to below average, peaking in the low to mid 30s for most of the area, remaining similar through Wednesday. The first such trough will pass overhead on Monday, leading to slightly increased cloud cover and isolated snow showers in the morning hours; the 12z CMC and GFS develop this into a more widespread 1-3 inch snow event, which currently does not have much model support but will be monitored for the potential for more widespread precipitation. The next two shortwaves will follow in quick succession on Tuesday and early Wednesday, with the model guidance struggling to handle which will be the dominant feature; the ECMWF focuses most of the precipitation on Tuesday’s shortwave, with at least 1-3 inches of snow affecting the area, while the GFS and CMC focus on Wednesday’s shortwave, with similar accumulations on Wednesday morning. Along with the timing uncertainty, the position and strength of any low pressure that develops also needs to be monitored considering the model guidance often tended to underestimate the strength and amplitude of shortwave troughs in the medium range with this winter’s pattern. These uncertainties preclude a higher confidence outlook regarding the exact timing, although there is at least a moderate probability of a light to moderate snow event between Tuesday and Wednesday morning with the exact timing to be determined; the 8-day outlook to be posted shortly includes a 40% chance of snow from Tuesday afternoon through Wednesday morning.