The Saharan Air Layer (SAL) is a mass of very dry, dusty air that forms over the Sahara Desert in northern Africa during the late spring, summer, and early fall and it can move westward over the tropical North Atlantic on a regular basis during the Atlantic Basin tropical season. SAL activity usually ramps up in mid-June and peaks in mid-August and sometimes the individual outbreaks can cover vast portions of the Atlantic Ocean and reach as far west as the central US and Central America. The SAL has unique properties of dry air, strong winds and warmth that can have a significant inhibiting effect on the formation and/or intensification of Atlantic Basin tropical systems - at least on a temporary basis. This outbreak of dry, dusty air is more widespread than usual and appears to have formed when strong winds from thunderstorms kicked up the dust to an altitude of about 20,000 feet in mid-June over the Sahel region of northern Africa.
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There is a surface low pressure system this morning near the Outer Banks of North Carolina and satellite imagery clearly shows an upper-level low spinning over the central part of the same state. This upper-level has resulted in substantial rainfall in recent days across the Carolinas and westward to West Virginia and southwestern Virginia and it is about to finally make a move to the next. As a result of this movement, showers will move into the DC metro region later today and likely edge into Philly later tonight or early Thursday and once the threat of rain arrives, it’ll likely stick around all the way into next week. In fact, another significant upper-level trough of low pressure is likely to push into the Great Lakes/Mid-Atlantic/NE US by the middle of next week and this will more than likely continue the wet pattern that is unfolding for this part of the nation.
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There are two important weather stories here at the end of the work week with continued very unsettled weather conditions in the Mid-Atlantic region and also tropical depression Cristobal which remains just inland over the Yucatan Peninsula region of Mexico at mid-day. The surge of summer-like heat and humidity into the Mid-Atlantic region on Wednesday was accompanied by severe thunderstorm activity and a stalled out frontal boundary zone will remain the focus area for showers and thunderstorms over the next 24 hours or so. Any rainfall in the Mid-Atlantic region can be heavy at times with the potential of localized flash flooding and any thunderstorm that forms can be strong-to-severe with gusty winds.
Meanwhile, tropical depression is parked inland over Mexico, but it is likely to move back out over the open warm waters of the Gulf of Mexico and take a turn to the north this weekend. There is a chance that Cristobal – the third named tropical system of this young tropical season – will close in on the north-central Gulf coastal region by the end of the weekend and all eyes from the Panhandle of Florida to Texas should remained focused on this threat.
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There are two important weather stories here at mid-week with a sudden surge of summer-like conditions into the Mid-Atlantic region and the development of a tropical storm in the southwestern Gulf of Mexico. The surge of summer-like heat and humidity into the Mid-Atlantic region will be accompanied by some strong-to-severe thunderstorm activity in some spots later today and tonight as a cold front approaches from the northwest. The highest probability of the severe storm activity will be to the north of the PA/MD border with one shot around mid-day and then another shot from later this afternoon into tonight. This front stalls nearby on Thursday and this could result in some heavy rain bands tomorrow and tomorrow night in the Mid-Atlantic region.
Meanwhile, Tropical Storm Cristobal has intensified in the overnight hours, but it may very well weaken over the next couple of days as it moves inland over Mexico. However, by the end of the week, TS Cristobal is likely to move back out over the open waters of the Gulf of Mexico and take a turn to the north this weekend. There is a chance that Cristobal – the third tropical storm of this young tropical season – will close in on the central Gulf coastal region by the end of the weekend and all eyes should remained focused on this threat from the Panhandle of Florida to Texas.
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A pre-frontal trough of low pressure will become the focal point for showers and thunderstorms late this afternoon and early tonight in the Mid-Atlantic region and some of the storms can reach strong-to-severe levels as they approach the I-95 corridor. A strong surface cold front will arrive in the same corridor late tonight, but it will encounter a more stable atmosphere likely not resulting in additional showers or storms. A much cooler air mass will arrive in the Mid-Atlantic region tomorrow night and Sunday and the month of June could actually begin with temperatures in the 40’s early Monday in some of the northern and western suburbs in the I-95 corridor. Elsewhere, unsettled weather continues across the Southeast US over the next couple of days and there can be some shower and thunderstorm activity in Florida on Saturday, but prospects for the 3:22 PM SpaceX launch at Cape Canaveral are perhaps a bit higher than they appeared yesterday. Looking ahead, there are indications that tropical activity may develop over the Gulf of Mexico in about a week to ten days and we’ll monitor those prospects in coming days.
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The “official” Atlantic Basin tropical season begins on Monday, June 1st, but nature itself has, of course, gotten off to an early start. This actually continues a trend for a pre-June 1st start to the tropical season as there has been a named tropical (or sub-tropical) system in each of the last five years. Tropical Storm Arthur brushed the North Carolina coastline about ten days ago and Tropical Storm Bertha became the second storm of this young season and came ashore on Wednesday in South Carolina. There are signs that the active and early start to the 2020 tropical season will continue as we transition into the month of June which begins on Monday. In fact, there are reasons to believe that the Gulf of Mexico region could feature a tropical storm or even a hurricane by late next week or during the following (second) week of June.
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The east coast of the US will be an interesting area to monitor over the next week or so in terms of the weather as we’ll see a tropical system just east of Florida in the near term and then perhaps a coastal low by the end of next week which could impact the Mid-Atlantic. Low pressure is now forming over the Florida Straits and it’ll intensify over the next 24-48 hours as it makes a move to the northeast over the relatively warm waters of the southwestern Atlantic Ocean. This system may come uncomfortably close to the US east coast in coming days and may play a role in the development of a coastal low by the end of next week in the Mid-Atlantic region.
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Another cold shot is coming to the Mid-Atlantic/Northeast US for the first half of the week and it could lead to more record or near record low temperatures and the possibility of frost during the next couple of late nights. As with the remarkable cold air outbreak this past weekend, attention should be paid to sensitive vegetation with this unusual late season chill. The end of the week will bring about a dramatic turnaround and summer-like warmth is quite likely on Friday as a “Bermuda-high” type pattern develops over the eastern US and Atlantic Ocean.
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A remarkable cold air outbreak is coming to the northeastern quadrant of the nation for Friday night and Saturday and it will be accompanied by a rain-changing-to-snow event for many interior sections of the Mid-Atlantic/Northeast US. This cold blast is likely to result in some of the coldest daily temperatures ever recorded both at the surface and in the upper atmosphere and, in some cases, monthly temperature records may be set. In addition, some spots might see their latest measurable snowfall in the Friday night/Saturday time period. Some interesting notes…in Philadelphia, there has not been a temperature in the 30’s on May 9th or later in more than fifty years (1966) and it could happen on both Saturday (9th) and Sunday (10th) mornings. In Buffalo, the predicted temperature of -40°C this weekend at the 500 millibar level would shatter the previous record of -35.7°C for the entire month of May. In Baltimore, there has not been a measurable snowfall in the month of May since 1872 and the latest on record there is May 9th (1923).
In addition to the cold and potential snow, winds will become an important factor as well on Saturday with gusts past 50 mph on the table in the Mid-Atlantic/Northeast US and this will raise the chance for scattered power outages. Looking ahead, the overall weather pattern that has resulted in multiple cold air outbreaks from Canada into the central and eastern US during the past several weeks is likely to finally break down during the third week of May, but we’ll have to endure some pain before we get to those expected warmer-than-normal conditions.
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Today is an unusually chilly day in the Mid-Atlantic region with temperatures far below-normal for the 6th of May aided by a low-level easterly flow of air and thick cloud cover. In fact, high temperatures today in much of the Mid-Atlantic region will be confined to levels rarely experienced for this time of year. Believe it or not, this air mass may pale in comparison to what is on the way for the early part of the weekend.
Quite an amazing outbreak of cold air will arrive in the Mid-Atlantic region and Northeast US on Friday night and the cold frontal passage may actually be accompanied by a rain-changing-to-snow event – even into higher elevation suburban locations just to the north and west of I-95. Saturday will turn out to be a very windy and unusually cold day in much of the northeastern quadrant of the nation with temperatures in the DC-to-Philly-to-NYC perhaps more than twenty degrees below normal. While Sunday, Mother’s Day, will remain colder-than-normal, the afternoon will become much more bearable when compared to Saturday as there should be far less in the way of wind and plenty of sunshine. Looking ahead, cold air outbreaks will likely continue into mid-May across the central and eastern US, but a pattern change to warmer looks like it’ll begin during the 3rd week of the month - and tropical season may not be far behind.
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