Unsettled weather dominates week
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*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.

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 rain or severe events, not publicly. To join this discussion and hear our updated thoughts from our team, visit The My Pocket Meteorologist Page by clicking HERE and select the “Premium Weather Forum” option.

Precipitation:

After a dry weekend, a cold front will move through the region on Monday 5/22 with showers and perhaps a few thunderstorms preceding the front as early as Sunday night after midnight, continuing into Monday morning. Severe chances are low given the timing of the front, and precipitation amounts should in most cases fall below 0.5″. There is the opportunity for some partial clearing later in the day if the front clears the region fast enough, and highest likelihood for that to occur will be in our western zones.

A complex series of low pressure will affect the region Wednesday night and Thursday with more rain expected in that time frame. The first area of low pressure is modeled to remain south of the region on Wednesday, and as that system slides offshore, a 2nd area of low pressure will move through the eastern Great Lakes. Some redevelopment is possible over Maryland that could enhance rainfall rates, making more of a steady rain event vs. lighter showers. We will handle this system as necessary in the shorter term updates, and weather alerts as needed.

All public/freely available maps will be updated when the threat is imminent for severe weather on the weather alerts page throughout the Spring and Summer: http://epawaweather.com/weather-alerts/

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

The GFS shows the cold front by early Monday morning poised to move through, but its advance may be slowed by high pressure offshore. Rain can be expected from later Sunday night and at least Monday morning, lingering into the afternoon farther east.

The same GFS run shows a primary low pressure moving through the eastern Great Lakes Wednesday night, with possible secondary low pressure redevelopment near the DelMarVa region. This could act to possibly enhance rainfall Thursday morning.

Pattern:

What looked earlier in the month to be a sustainable warm pattern to close out the month of May has now become a much above normal period Wednesday through Friday of this week, that will be abruptly ending Friday night. Unlike cool snaps prior, and given the time of year we are moving into, this cooler period will not be nearly as cool as prior events. We will, however, enter the vast majority of the month near to slightly below normal. This translates to temperatures generally in the 68-75°F range for most of the region, not 50s and lower 60s as seen earlier in the month. The only caveat to all of this is Memorial Day weekend looks to at least start on the cooler side, and although pleasant air temperatures, not the best weather to unofficially begin the beach season.

The driver of the cooler weather in the early part of the month was a near-unprecedented (for May) negative North Atlantic Oscillation (-NAO) and resultant omega blocking pattern, centering a ridge over the North Atlantic, and keeping a deep trough in the east. The NAO will turn negative once again as the ridge over the region moves out, but will not be as anomalously strong or negative as its predecessor. The Southern Oscillation Index (SOI) rollercoaster also makes for a tougher long range forecast with amplification of the jet still a prominent factor. Still, a trough will set up for much of the remainder of May that will allow temperatures to remain near to at times slightly below normal. As stated above, it translates to refreshing Spring-like temperatures rather than extreme heat, or the polar opposite, extreme cold.

Heading into the month of June there will be ups and downs continuing, with nothing sustainable beyond 5-6 days. The month will start out slightly above, and then transition to above normal temperatures for at least the start of the 2nd week of June, but troughiness can still show up throughout the month at times, keeping temperatures in check (back to near average) from time to time. Biggest takeaway is although “slightly below average” sounds “cold” it is far from it. Normal highs as we head into June start heading toward the 80° mark, so even a slightly below pattern occasionally will still produce warm temperatures. By the end of June, the temperatures we think average out to slightly above average looking at the month as a whole. Precipitation like May should average near normal.

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

The GEFS (GFS ensemble) shows below average temperatures at 850mb over the region though much of the remainder of May, but this translates to only slightly below average temperatures at the surface. The -NAO (Greenland block) is the driving force here.

The same GEFS (pictured) and most other ensemble groups suggest that as the -NAO loses its grip in the first week of June, combined with less amplification, a more zonal flow develops and temperatures respond by heading to slightly above average again.

EPAWA 1-5 day temperature departure outlook

EPAWA 6-10 day temperature departure outlook

EPAWA 11-15 day temperature departure outlook

EPAWA 16-20 day temperature departure outlook

Below is a look at the latest soil moisture anomaly, which is an integral part of drought outlooks. Reds and oranges represent below normal soil moisture levels, greens represent above normal values.

This image will automatically update daily.

The My Pocket Meteorologist text alert program gearing up for the upcoming severe season. Seasonal options will be available beginning March 31st. Click on the image below for more information.

Forecaster: EPAWA Meteorologist Bobby Martrich
Discussion last updated:  Friday May 19th, 11:30am