Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Tue Sep 17 2013 Valid 00Z Wed Sep 18 2013 - 00Z Fri Sep 20 2013 ***Periods of rain expected for Northwestern U.S.*** ***Unsettled weather for the southern tier states*** ***Autumnal airmass over the Eastern U.S.*** Additional shower activity is expected for much of the Pacific Northwest and into the Northern Rockies as an upper trough over the northeastern Pacific slowly moves inland. Onshore flow behind this system will further help the development of scattered showers in orographically favored locations. A relatively strong surface low is expected to develop over the Northern Plains late on Wednesday with additional precipitation expected to develop across northern Montana and into North Dakota. For southern Texas and southern New Mexico, plenty of moisture in the atmosphere will aid in the development of scattered showers and thunderstorms across the region over the next couple of days. Isolated to scattered convection is likely for the Florida peninsula as a stalling cold front allows for wind convergence and lift in the lower atmosphere. More delightful weather is in the forecast through mid-week as a pristine airmass from Canada remains in control. Some northern locations could even call this weather cold, with frost advisories and freeze warnings in effect from interior parts of Pennsylvania to central New England. While seasonal to below average temperates are expected to persist across the Northeast into the mid Atlantic region through Wednesday, a moderating trend is expected by the end of the week as southerly flow resumes on the back side of the high as it moves offshore. Hamrick Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_wbg.php