Pattern shift is on the horizon, but
the changes will not be immediate Get The Forecast!

Technical discussion below will feature two (2) subcategories: Precipitation, and Pattern Discussion. Storm possibilities will be discussed exclusively in the Premium Forum with intense model analysis leading up to any major snow, ice, or rain events, not publicly. To join this discussion and hear the updated thoughts from our team, visit The My Pocket Meteorologist Page by clicking HERE and select the “Premium Weather Forum” option.

Precipitation:

OVERVIEW: A rather fast-moving and relative zonal pattern is currently contributing to the relatively drier pattern that we’ve seen over the past week or so. After mid-month, we do expect an uptick in precipitation chances, and given the expected temperatures for the 2nd half of this month, could lead to multiple wintry events, most notably during the last week to 10 days of January and first week of February. Overall, precipitation departures from average for the month as a whole we maintain will be near to slightly above average with a warmer than average first half of the month skewing the mean monthly temperature departures.

The first chance for precipitation listed in this outlook is with an expected cold front that will bring rain to most areas Friday night, with a change to snow possible in higher elevations of North Central and NE PA before ending either very late Friday night or very early Saturday morning. Minor snow accumulations of a coating to an inch or two are possible mainly in elevations above 1000 feet, and up to 3″ is possible above 1500 feet, but restricted to those areas mentioned. The second precipitation chance listed is last week’s storm signal for next Thursday/Friday. Uncertainties exist with track and evolution for this system with the pattern shifting at this time to support overrunning or southwest flow events (SWFE) with not only this particular signal, but with subsequent storm threats as well. These types of systems support low pressure sliding off to our immediate south or over the region, and depending on track, produces snow and/or mixed precipitation on the northern side of that low track. Nearest the low pressure and south of it would feature rain. The system is currently not modeled to be a large storm, but we will monitor for changes. The remaining precipitation events listed in this outlook are Winter storm signals, which offers no guarantees of wintry weather at any given location, but the probabilities of occurrence is higher than usual in the time frames listed. The first Winter storm signal falls between the 25th/26th, where another similar type of system (SWFE) could affect the region. A second Winter storm signal was also listed for the 30th/31st period. Given the range, these two remain only signals for now, and will be monitored over the next 10+ days. Additional signals are likely during the first week of February, but not at a range where confidence is high enough to list individual dates/periods of activity. This active period is likely to continue into the first several days of February.

The month of January we project will finish with near to slightly above to above average (+0.0″ to +1.0″) precipitation departures for the month as a whole with moderately high confidence. Snowfall is above average year-to-date with the exception being far SE areas of southern DE and coastal Southern NJ, and snow chances look to pick up during the 2nd half of January, especially the final week to 10 days of the month. Early look at the month of February suggests a slightly above average precipitation (+0.5″ to +1.0″) anomaly for the month as a whole, with slightly above average snowfall. It should be noted that February yields the most snow of any month in the year climatologically speaking at most locations.

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Breakdown of precipitation departures from normal over the next several weeks:

January 17th – January 23rd:  Near to slightly below average

January 24th – January 30th:  Near average

January 31st – February 6th:  Near to slightly above average

February 7th – February 13th:  Near to slightly above average

February 14th – February 20th:  Slightly above average

Public/free available maps will be updated when a threat is imminent for significant weather on the weather alerts page throughout the year:  http://epawaweather.com/weather-alerts/

Note:  Any image in the outlook is clickable for larger viewing  

A cold front moves through Friday night that will bring rain for most areas, but as colder air works in aloft late Friday night, rain changes to snow in areas of elevation, with the highest amounts over 1500 ft.

The next chance for precipitation in the outlook is for the next Thursday/Friday period, and could bring overrunning snow/mix to parts of the region. This system doesn’t look like bigger storm at the moment.

Pattern:

OVERVIEW: The Madden-Julian Oscillation (MJO) is currently in phase 3 in the Indian Ocean, and is expected to move back into the COD is expected by early next week. The MJO is expected to largely remain in the COD for much of the remainder of the period, with a possible eastward shift to phase 6/7 in the Western Pacific in early February. A Sudden Stratospheric Warming Event (SSWE) has occurred on January 5th, 2021 that on that date completely reversed the zonal winds from westerly to easterly at the 10hPa level in the stratosphere. The effects of this will result in a major pattern shift in the Northern Hemisphere. Intense warming over the polar regions are displacing cold air from those polar regions and forcing it southward to the mid-latitudes. There is a lag period involved with such a dramatic change, coupling with the troposphere, and effects to our sensible weather patterns in the US, and we project these initial changes will occur at the end of January week 3. The result will be a colder and snowier pattern that develops for the last 10 days of January and to begin February.

Our pattern during the past few weeks have been largely influenced by a Pacific jet extension between Eastern Asia and the NW coast of the US, resulting in slightly above average temperatures. This jet extension contributed to the persistence of a Gulf of Alaska low presence, which despite blocking in place at the time, prevented the cross-polar flow necessary to bring colder air into the continental US. A positive Eastern Pacific Oscillation (+EPO) resulted from the lowered heights across most of Alaska during this time. In response to East Asian Mountain Torque in eastern Russia, China, and Mongolia observed to flip from positive (+EAMT) to negative (-EAMT) over the past few days, the accelerated flow off of the Himalayas is cut off, and the Pacific jet extension will recede/relax around the 19th. As a result of this relaxation, the void created will allow ridging to surge into Alaska just after the 19th. This produces a cross-polar flow (-EPO) that finally allows colder air to surge into Canada and the US. This transition occurs on time with what we’ve been projecting for the past month each week, without moving from that projected date of arrival. It should be noted that although an extended below average period is expected, it is time limited to two weeks or so, and it is NOT a brutally cold air mass. Sensationalistic media headlines of “polar vortex” and “brutal cold” mentions should be ignored. It is what it is, and that is a true pattern shift for about two weeks to a colder than average pattern during the climatologically coldest time of the year. Another such pattern shift we project will occur during the 2nd half of February, continuing through at least early March. A pattern change refers to significant changes in the Northern American continent with regard to the 500mb height anomalies and the resultant change in temperature patterns. Although snow chances/probabilities increase during this period, accumulated snowfall is not a metric to define any pattern change.

Average highs and lows are near annual climatological minimums, and normal high temperatures are in the 33-40° range from north to south across our coverage region. Any slightly above or below average stretches listed in the outlook will be relative to seasonal norms at that time. The forecast period over the next 5-6 weeks will begin with a zonal pattern for a few more days, and slightly above average temperatures will result locally, but a pattern shift occurs around the 21st/22nd as we have been advertising for 4 weeks. Snow chances as outlined above in the precipitation discussion are more likely to occur in the last 10 days of January and continuing into the first week of February. The final 10 days of January turns colder than average, largely due to recent stratospheric and forthcoming Pacific jet and EPO changes, and remains that way into early February. A turn to slightly milder than average from late week 1 of February through about the middle of the month as a result of a combination of the EAMT flipping positive and expected tropical forcing (MJO) at that time. A turn to cooler temperatures and with time, slightly below average during the 2nd half of February into at least early March. It is important to note that cooler/warmer periods will be relative to the time of year we are talking about. For example, slightly above average temperatures listed over the next few days comes at the climatologically coldest time of the year, and still yields highs in the lower 40s in most areas with respect to the normal/average highs at that time, and in February, normal highs and lows only BARELY moderate. Use the average highs/lows chart above that provides a point of reference for what average is for each corresponding date, with the examples used for the Philadelphia region.

Snowfall: Several chances for snow or threats thereof exist for the last 10 days of the month of January, and at least the first several days of February. Threats will appear every 3-4 days on average during the expected 2 week period of the pattern shift. Additional chances reappear during the 2nd half of February.

For the month of January, we expect slightly above average temperatures (+1.0°F to +3.0°F) when the dust settles January 31st with high confidence. This is a bit misleading however, as an anomalously warmer than average first three weeks of the month outweighs and skews the mean monthly temperatures compared to the final 10 days or so of the month, which are likely to turn colder than average during that stretch. Looking at the month of February as a whole suggests a near average month (-1.0°F to +1.0°F) using long term climate models and seasonal/global observation trends. A trend even colder than this is possible with time, but near average was listed largely due to uncertainties with stratospheric warming and downstream effects with regard to our sensible weather changes.

Note:  Any image in the outlook is clickable for larger viewing

The chart below provides a point of reference for what average/normal snowfall should be at several select climo locations within our region. This is the same chart used in the Sunday Weather Weeklies videos.

A colder pattern settles into the region late next week, and will likely maintain through early February. 500mb changes on the loop below show a relaxation of the Pacific jet, which induces a -EPO with time.

Outlook table last updated: Friday January 15th, 11:45 AM. Next scheduled update: Friday January 22nd.

This is a weekly updated public long range guidance product from EPAWA. For daily long range updates and more detailed updates M-F, please join the EPAWA forum. More information/sign-up at: http://epawaweather.com/my-pocket-meteorologist/ 

*Indications of above or below average temperatures in the table above are relative to what is considered “normal” using data collected over the long term for a particular date. This is collected and maintained by the National Climatic Data Center in conjunction with the National Weather Service actual data from previous years collected at official ASOS/climatology stations across our coverage area. Also note that as time moves forward into a different period as shown above, average temperatures for those dates will also change. See the example below using Philadelphia, PA as the climo station:

The departure from normal uses the average temperature for the date, averaging temps over 24 hours for any given location, using both high and low temperatures hourly during any particular day. This outlook determines warm vs. cool periods relative to normal temperatures.

Images below are clickable for better viewing

Days 1-5 – confidence VERY HIGH

Days 6-10 – confidence HIGH

Days 11-15 – confidence MODERATELY HIGH

Days 16-20 – confidence MODERATE

Below is a look at the latest Soil Moisture Anomaly for the Contiguous United States. This is used by NOAA/CPC for the purpose of drought monitoring and drought outlooks.

This image will automatically update daily.

Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated: Friday January 15th, 11:45 AM