A large upper-level trough over Mississippi River valley will be taking a negative tilt. A deep southerly flow ahead of it, will transport an abundant amount of moisture from Gulf of Mexico and Atlantic moisture up the Eastern Seaboard between Thursday and Friday. At the surface, a cold front with couple of low waves running along it, will be a focus for strong convergence and instability. This will enhance an area of heavy rainfall with embedded thunderstorms. Widespread rainfall totals of 2 to 3 inches are likely from Pennsylvania southward to Georgia. Localized higher totals are possible, especially where thunderstorms train.
The 0z GFS showing 2″ -3 of rainfall for the Mid-Atlantic Region (image courtesy of College of Dupage)
For the New York City Tri-state area, a strong high pressure centered over Western Atlantic, should keep most of the rain away until it begin shift east on Friday. Showers will likely increase from west to east during the day Friday, as the cold front slowly nears the the area. Some of these showers can produce some downpours. Then a wave forming along cold front, will enhance more rainfall Friday night. A strengthening southerly low-level jet and high precipitable water values 1.50″ will produce moisture convergence and elevated instability for more heavier showers and scattered thunderstorms.
The 0z GFS tonight showing preciptable water values over 1.5″ over the NYC Tri-state area Friday night
When all is set and done early Saturday morning, rainfall totals will likely be between 1 to 2 inches for much of the tri-state area. Locally higher totals are possible in spots where thunderstorms occur. his could cause some minor flooding in poor drainage areas. As well some renewed flooding across streams and rivers. But major flooding is not expected. The good news is that rest of the weekend is look dry and comfortable with high temperatures in the lower to middle 70s.
So far, this Spring in the Tri-State Area, we have seen more cooler weather, dominate and warmer periods more transient. Last week was another example with Tuesday and Wednesday will very cool, with east winds bringing marine air off the ocean. While Thursday we saw temperatures warm into middle to upper 70s, away from immediate coast. According to a tweet from the National Weather Service in Upton, NY, January to April 2014 averaged below normal for Central Park. The last four-month period to do that,was from April to July 2003. Later this week, looks like more swings with temperature, with the warm front position each day very important. So when will the warmth be here to stay for good?
First we need too look at what has been causing all this huge swings temperatures. The first, most persistent feature, are the above normal sea-surface temperatures in the Northeast Pacific. This enhances a stronger ridge over parts Northeast Pacific Basin, usually ahead of deep trough or low positioned near the Aleutian Islands. This pattern is known as EPO (Eastern Pacific Oscillation), in its negative phase. This is a feature that has been there, since the late fall of 2013.
Persistent pool of warm SST anomalies over the Northeast Pacific Ocean
The -EPO ridge often shifts east into West Coast of North America. However, with the lack of sustained blocking on the Atlantic side the pattern remains somewhat progressive. The -EPO ridge shifts east into west coast of North America transforming into +PNA pattern. Then when the next Pacific storm moves in Aleutians/Bering Sea, the -EPO ridge reloads with enhancement from the warm sea-surface temperatures over the Northeast Pacific.
500mb mean height anomalies from January-April 2014 showing an -EPO pattern dominating over North America
A moderate to strong, El Nino is forecast later this summer. The means we are likely looking at a warm East Pacific Chances are this warm SST anomaly will also persist through the summer and beyond. So the -EPO likely won’t be going away for good, anytime in the near future.
CFSv2 ensemble forecasting sea-surface anomalies +1K to +2K for Nino 3.4 region
Warmer than average temperatures over also in the lower stratosphere has supported the -AO/-NAO blocking to develop. This has resulted keeping deeper trough over SE Canada or the Northeast for much of this Spring. Although there has been a tendency for the -AO/-NAO blocking to be transient, because there has been a parade of Pacific storms crashing into West Coast. So were not able to lock cooler than normal pattern over the Eastern CONUS either. Instead, we see more volatility with huge swings in temperature.
70mb temps in lower stratosphere running above average, late March/early April
The models show the AO/NAO rising next two weeks. So that will allow the for another intense, warm surge late this week for Eastern US. However, models now trending stronger with trough over SE Canada. This trough will produce confluence or high pressure over New England and likely force the warm front to stall to our south on Thursday. Keep most of the area in cool, marine airmass with chance of a showers.
GFS showing the warm front remaining south of the Tri-State area on Thursday. Temperatures in 50s over Northern New Jersey, New York City, and Long Island.
The trough will start shifting east on Friday. Allowing the warm to make more progress northward. We could see big difference in temperatures and sky conditions from SW to NE across the area. More widespread 70 or 80 degree temperatures are more likely this weekend. With a little more sunshine and showers and thunderstorms more widely scattered.
The MJO is now active and will be propagating into phase 2 next week. Which supports another period cooler and wetter weather over the Eastern US, sometime between May 15- 20th period. Beyond that, MJO might get into phases 5 and 6. Which supports longer period of above normal temperatures for much of Central and Eastern CONUS for late this month. With El Nino growing the summer, the MJO is likely to be more active and have some influence on our weather pattern.
15-day ECMWF ensemble forecast for MJO propagate into phase 2 next week.
As we go later in Spring and Summer, the cold impacts of -EPO/-AO/-NAO gradaully decrease. The supply of cold air begins to run out. The configuration morphs into a pattern that supports warmer west-down slope flow in our region. So will probably warm periods will last longer. However, we still have monitor for troughs over SE Canada or New England to force warm front to stall south of region or to push backdoor cold front in our region. Until mechanisms producing -EPO/-AO/-NAO patterns from to time start to change.
Torrential downpours trained over the region late in the afternoon and overnight. Widespread rainfall totals between 4″ to 6″ fell along the I-95 corridor between the immediate New York City and Philadelphia metro areas. This caused moderate to major flooding in low-lying areas and along streams and rivers. A mudslide also buried two cars in Port Washington on Long Island.
Central Park in New York also set a daily record rainfall record of 4.97″ for April 30th (storm total for two days is 5.12″). This was also the 10th wettest day recorded. Daily rainfall records were also set in LaGuardia Airport 5.26″ and JFK Airport 4.68, Newark Airport 5.24″. LaGuardia and JFK had their 4th wettest days record. Newark had their 6th wettest day recorded.
An omega block this week caused a large cut-off low to from over the Central US. “A cut-off low” is a storm that breaks away from jetstream and spins around very slowly, due weak steering currents. These lows sometimes bring prolific rainfall totals to our area. Especially during the spring or summer. So much moisture is transported from the Gulf of Mexico and the Atlantic and then is forced to rise and condense into very heavy rains.
Miguel Pierre Weather 